electrical service to a shipping container workshop
It sounds like you have things well thought out here as far as electrical matters go.  A couple of notes:


You may wish to bury a fat conduit (or two -- I'd use 2" or 3" Schedule 80 PVC for this) in the trench as well as the PEX line and the UF cable, or better yet, bury some conduits and use one for the feeder to the container-building.  This way, you don't have to dig up or abandon the UF later if you need MOAR POWAH at your workshop.
You'll want to have your ground rod checked by an electrician to make sure it is less than 25 ohms to ground (for adequate lightning protection of the electrical system).  If it isn't, you'll need to install a second one at least 6' away and bond them together with the Grounding Electrode Conductor.
You are correct in that the container should be earthed.  If you don't plan to move it later, an exothermic welding kit, used properly, will make a permanent copper-to-steel bond between the bonding conductor to the container and the container chassis -- I would bond to a container frame rail to minimize the risk of the exothermic weld accidentally perforating the container wall sheets.
A strike terminator (lightning rod) is a good idea, especially if the container is the tallest thing in the area.  Note that the lightning rod needs its own earthing electrode -- it cannot share an earthing path with the power system to avoid excessive common-mode voltages getting induced onto the power system and frying things.
The minimum size for Grounding Electrode Conductors in the NEC is 6AWG -- bare 6AWG copper wire is easy to find, and can be directly buried without issue.  This wire needs to run between all the ground rods, the container chassis bond point, and the panel ground bar.