I would consider the design like any other room. Just be mindful of the required egress, panic hardware, etc. if you have large equipment.
I've done this many times for aggregate crushing plants, those are the pertinent issues. If the user is considering using the rear swing-out doors as the only access point, that will not fly because you can't use panic bars on them. So you will need to cut in at least one door. The other thing that has changed is that people often want to put in a window, desk, chair, console etc. because they have seen that done in the past. That too is now problematic when you consider the Arc Flash Safety issues, resulting in having to create an interior barrier to keep the electrical equipment in a separate room and then another egress point for the "office" area.
Side note: From my experiences, the steel used in shipping containers is EXTRA hard, it will tear up cutting wheels, sawzall blades etc. The only effective way to cut in doors and windows is using a plasma cutter.
Another tip since you said you will have VFDs inside. Shipping containers in the sun act like "hot boxes", like those used in POW camps to torture prisoners. Insulate the HECK out of it inside, and paint the outside with heat reflective paint. I used stuff called 'Cool Coat" that had ceramic spheres in the paint which would reflect away about 80% of the solar radiation. It worked REALLY well. We would assemble the gear (usually MCCs) inside and sweat our gonads off, then the painters would show up, spray the outside with that stuff and the interior temperature would drop 30 degrees (from 120 to 90, but that's a big change). After that first experience we learned to have it painted FIRST, even if it meant having to repaint if after damaging the paint.