Record default command used to run repo2docker in Dockerfile

[yuvipanda]

When running repo2docker, we might want to record the 'default command' the user wants to run there. This is useful when running the image outside of repo2docker itself. But we have to be careful about what this means for caching.

Similar to #545 and split from #533 (comment)

[yuvipanda]

We might wanna separate this out from the runtime command, maybe - but wanna do the 'right thing' by default as well.

[betatim]

The idea would be to record the build time repo2docker https://github/org/repo my-command-here arg1 arg2 in the Dockerfile by having CMD my-command-here arg1 arg2 (or ENTRYPOINT? Tim can't remember when to use which). Replacing the default which we put in right now independent of what command is specified by the user as argument to repo2docker.

[manics]

It's been a couple of years, do you still think we should add this?