It's called an "on-demand" water heater.   I found another "on-demand" system in my Peruvian hotel last April that used electric coils in a large shower head.   But the idea of 220 volts in my shower is a bit unsettling.   I also learned that for safety concerns, some cultures aren't yet comfortable with gas in the house to cook or heat.   They prefer electricity.   However, I think a wall mounted, propane powered, "on-demand" water heater is the best solution for an efficient container dwelling.  

I have found models available for USD$300-700.   Most have electric ignition but one uses two D size batteries that supposedly last 2 years (Aquastar 125 = USD$600).   There are also solar versions that preheat the water.   Sizes are all similar, in the 30x18x10 inch range (75x45x25cm) and 40-60 pounds (18-28 kg).   Installation would require the propane connection and a 4-5 inch (10-12cm) hole cut for venting to the outside.   More self - sufficient water systems would involve building pressure and even collecting and storing some of the water needed.   Small electric pumps are readily available to bring water up from wells, river, or lake sources.   Some also pressurize it to create a shower.   Hand pumps are a simpler option.   One product called the "Hot Man Camper Shower"  (from Wyoming River Raiders) is essentially a hand pump garden sprayer made in stainless steel.   It's a 2.5 gallon tank (9.5 liters) with a pump, temperature gauge, hose, shower head, and a valve.   It can be placed directly on a heat source (such as your propane kitchen stove) and delivers five minutes of continuous hot shower.   And at $100-120 it's a lot cheaper than the gas heater and offers pressure and plumbing.

Water storage then becomes an issue.   I imagine using a large plastic container with a removable top.   These are often available from farm supply stores.   I have seen many sizes in South America molded from durable polypropylene and used for roof top water storage.   How much water you need or want to store will largely reflect the situations you intend to encounter and how self-sufficient you want to be.   If you have an "on-demand" water heater and municipal water with pressure then you need no storage.   Add a good filter to your sink and you don't even need to store drinking water.   But I want to expect situations where water is pumped, delivered, or collected from rain. 

A good awning cover could be designed to send rainwater to the large plastic container for pumping later into the shower or sink.   And the same container could accept water delivery or be filled from other sources.   For hand pump pressurizing, however, a smaller container would be better.   Three to five gallons is plenty of water for washing or showering and many people can do with half that after some practice. The idea of using the "Hot Man Camp Shower" as a cheap source of pressurized water for doing dishes (cold water) or heating it and taking a shower seems like a simple solution.

Frequent refilling of the 2.5 gallon tank might become tedious, but the cheap cost plus flexibility could make the choice over more expensive options that still require plumbing and pressure. Plumbing.   You have to drain out the used water from bathing and washing.   And I get tired of fussy plumbing that won't accept a few coffee grounds or bits of vegetables once in a while.   Go big.   Easier to clean and avoids clogging.   Two inch (5cm) diameter PVC plastic pipe is cheap and simple to use.   In my design, I only have two water outlets (shower + sink) and two drains that quickly merge and exit the side of the container.   Raising the floor in the bathroom area about 10-12 inches (25-30cm) allows working space for pipes and enough incline for water to drain out well.   A factory made fiberglass shower unit solves a lot of plumbing, draining, and water sealing efforts.

And I can get one for less than $200 at the local building supply store.   I'd anchor it to the wall and brace it with floor to ceiling steel or wood beams.   As in any efficient home, water fixtures are grouped to allow minimal running of pipes.   I'd put the shower back to back with the sink.   I recommend putting all utility access/drains/vents along the same side of the container in case location requires blocking one or two sides.  Another thought:   it may not be possible to continuously drain out water.   For situations where you need to be a bit more discreet, a 20-30 gallon reservoir that can be emptied with a valve might be desired.   In this case, raising the floor and planning the drain flow gets more critical.

Toilet.   This feature still has me searching for alternatives.   Great composting toilets exist that use no electricity or a little (12volts), no plumbing, very easy installation, claim no odor, and require minimal maintenance.   And they're long time proven in boats, RV's, and summer cabins.   However, they start at $900 or more (SunMar Ecolet, Biolet). 

I think they are ideal, but the cost is extreme and I haven't yet found many options.   One toilet designed for river rafting trips offers a sealed box and a normal looking toilet seat for USD$120 (another product from Wyoming River Raiders).   It claims "50 user days" which seems like a long time between emptying (especially in a closed container!), but the cost and simplicity are there.   The toilet choices I¹ve found so far either give you an expensive fiberglass chambered system with a bacteria rich mixture to break down and absorb wastes, or a glorified zip-lock baggie and bucket set-up.

Propane.   Readily available in most of the world, cheap, and relatively safe, propane offers the container home a fuel source for cooking, refrigeration, and heating both water and air.   Good steel propane burners are widely available in camping stores for USD$75-200 depending upon features.   I¹m interested to learn what is required to convert a standard compact gas oven/range to use propane.   My cooking skills are primarily stove top based, but having an oven could be nice. 

Propane powered refrigerators are less common and therefore more expensive.   Figure USD$400 or more (unless you can find a used one) for a compact unit that will fit under the kitchen counter.   Many models can use both propane and electric power which would be my choice when steady local power is available.  I found one refrigerator that runs very efficiently on kerosene, but kerosene stinks, especially in a small space. 

I'm not going to spend much time here considering propane space heaters.   After 17 years in Chicago winters, I don't mind when temperature drop into the 50'sF (10C) and even lower at night.   That's what sweaters, warm blankets, and girlfriends were made for.   But there are numerous inexpensive propane heater units that bolt to the wall or free stand capable of warming a shipping container.   Just be careful on placement for fire safety.   The last propane appliance would be the gas "on-demand" water heater previously discussed.   I recommend 2-3 smaller propane tanks for easier filling and transporting instead of a single big one.   I considered a big tank secured in a steel closet located near the door for east access and filling, but more than likely you will need to carry a couple tanks some distance to get propane.   Multiple tanks also means you can have one tank dedicated to cooking and another to supply a steady trickle of gas to the fridge and water heater. 

Again, group the gas appliances closely along one wall to simplify the pipes and access.   Please see the links at the end of this article for a good website offering numerous propane products and information. Ventilation.   For safe use of propane appliances and fresh air in general, multiple vents are smart.   A hood and vent located directly over the stove will be a necessity to take out cooking smells (after that Cajun blackened redfish dinner!).   A good one with a decent fan can also go a long way to moving air through the whole container.   A 12 volt fan won't do much, but a good hood design and placement can do wonders for directing cooking odors.   I imagine a more powerful fan in place and used when  electricity supply permits.   The toilet and gas water heater have vent pipes included requiring 3-4 inch (8-10cm) holes to the outside.   Neither need fans but a 12volt battery fan for the toilet is an option.   I don't know about a need for venting the propane fridge yet.   For all vent openings to the outside (and drains for that matter), some method of closing and securing will be necessary during transport of the container home.   Any roof vents also need a rain cover. 

The roof of a shipping container seems to be the only weak part of an otherwise bomb-proof structure.   I've walked on several and the sheet metal flexes and warps under foot.   Because the four sides carry all the weight on heavy steel columns, the roof doesn't need much support.   However, if the roof is to be used as a deck area or have holes cut for light and venting, some re-enforcement is needed.   I'd simply weld square tubing on the inside like joists (every 24 inches or 60cm) and then weld the flexible sheet metal to these joists from inside.   The same steel square tube can make frames around larger holes cut in the roof for vents (24x24 inches) or skylights.   During transport, the holes can be covered with sheet steel and bolted closed. 

Furniture.  Endless options here depending upon personal tastes and building skills..   I recommend minimal to begin and adding as you define needs.    Considering the space as if on a sailboat is a good approach.   Transportation will cause bumping and tilting of the container, so latches on all drawers and cabinets are necessary.   Everything should have "lockdown" capability including dishes, tools, appliances, and fixtures.   I imagine welding "L" brackets to the steel walls and attaching any wood cabinets, counters, cupboards, or tables to these.   In my sketches, I put a table hinged off a narrow counter that flips down for more space.   I like big tables and maintaining some horizontal surfaces is good for me I'm a pile person with magazines and files and mail and whatever else I drag home.   Long narrow counters are great.   Eight and a half to ten feet of height (2.6-3m) in a typical shipping container offers a fair amount of storage above head level.   Under the bed is another big storage space.   Beds can be folded or converted into couches, but I chose to make the bed at seat height for use with the table next to it.   And I'd put large drawers on plastic glides (not wheels) under the bed that can be pulled out and hold quite a bit.   I¹ve been a furniture designer for ten years so I could fill pages with design plans, but that's the fun part of customizing this container home. 

I want to map out the necessary utility systems, make a few furniture recommendations, but let the readers do their own layouts.   Anyone who seriously considers making a home from a shipping container probably needs little advice about cupboards and tables.