GFCI protection for multiwire branch circuit in shipping container workshop
Since you have a metal structure and metal boxes, use metal conduit specifically EMT. Buy a bender and it's not that hard to work with.  The EMT itself is a valid ground path, and you will not need to run a ground wire at all.   Attaching the metal boxes and EMT clips to a metal building is icing on the cake. I have exactly this situation and I never run grounds, and there's never been the slightest doubt as to the reliability and ampacity of grounds.

Price 2-pole GFCI breakers for the panel you are using.  You may be in for a shock.  Your scheme of using MWBC will absolutely require a 2-pole GFCI.  ThreePhaseEel's advice of "just run one more neutral wire and get out from under the exotic breakers" is very good advice.

Using the same circuit for both 120V receptacles and also a 240V receptacles, I'm not a fan. Generally if an American appliance wants 240V, it's because it needs that much power, and so it needs all of it.  THHN is cheap and containers are not large, so just run an extra pair and be done with it.  

Remember the 310.15 derate rules: you can have 9 active conductors in any conduit of any size (before having to upsize wire per the derate).  Grounds never count, nor do neutrals in split-phase circuits (including MWBC)**.  So if you run a 240V circuit, a 120+240V MWBC, and two plain 120V circuits, you are at 8.  I often will lay two conduits next to each other if I expect to exceed 4 circuits. 





** Neutrals don't count in split-phase because they only carry differential current: Suppose a neutral is carrying 15 amps, and one hot is carrying 20, that means the other hot is carrying 5.  The heat from 2 wires carrying 15+5 amps is less than the heat of one carrying 20A, so those 2 wires get counted as one.  Heat is a function of current squared.

You ask, why 9? In a 3-phase "wye" configuration, neutral doesn't count for the same reason.  So three "wye" circuits  add up to 9 conductors that count (12 wires in 3/4" EMT conduit since conduit=ground). At 277V you can put 277*20*9=49860W - frickin 50KW, enough to light up a CostCo.