Dockerfile heredoc progress bars

[jedevc]

Hello again 🎉

This is a tracking issue for following up on this comment by @tonistiigi:

For the follow-ups I think there are some cases that need to be tested/added for the heredoc parsing with shell rules. I think there are some unhandled cases atm. Another thing to look at is what message to have in the progressbar so it gives the best indication for the user.

Essentially, buildkit should produce "useful" messages for heredoc commands.

There are probably a few aspects of this:

• File creation in a scratch container (need a message to indicate what files are being created, this currently doesn't exist at all)
• File copying in ADD/COPY (need a message to indicate the destination of the file, this kind of currently works)
• Running heredoc contents in RUN (need a message to indicate what is being run, and it would be very nice to give some indication as to how - since there's a couple ways that heredocs are created and run).

I'll hopefully have some time to take a look at this (and any other follow-up issues that end up being made) at some point this week 👍

[tonistiigi]

File creation in a scratch container

This can be [internal] preparing here-document

File copying in ADD/COPY

Yes, this mostly is ok already. Could consider normalized <<here-doc or smth like (inline) but not sure if improves.

Running heredoc contents in RUN

This is the trickiest. If there is a (long) command then I think we can just show it as is. But if it is just a script/shebang then I think we should try to find some words from its contents. python script: print("hello")... ?? Not quite sure.