Consider removing debug stage of dockerfile

[tpoliaw]

This should be remove at some point because AFAIK this is not being used

Originally posted by @ZohebShaikh in #345

[coretl]

@hyperrealist what (if anything) will go into the copier template Dockerfile in the future when you've extended plandev to work with FastCS? Should we just remove the debug stage now, or wait until we have something to replace it with?