What tool(s) are appropriate for cutting a door opening in shipping container?
There are many tools for cutting steel.  In addition to what you have suggested you could use:

A &quot;sawzall&quot; type reciprocating saw.
An oxy-acetylene torch.
A plasma cutter.
A body grinder with a cutting disc.

If you are good at following a marked line freehand then you should be able to just cut away and get the result you want.  If your cuts tend to drift, you might put together some sort of guide to keep the cuts straight.
(Since you asked...)
If I had to choose one, for example if this was my project, I'd probably opt for the plasma cutter route.  Plasma cutters cut through sheet steel like a hot knife through butter and leave a very clean edge.  The biggest con with them is that it can be tricky to keep a straight line.  But a straightedge of some sort will fix that issue easily.