Improve progress and history messages for heredoc-related commands

[jedevc]

This resolves #2167, following up on providing better progress messages for heredoc commands.

To summarize:

• File creation in a scratch container gets a nice internal progress message
• Simple heredocs (fake and shebang, but not complex ones) get a summary of the first line of the heredoc
• The commit message for copied files now includes a heredoc (instead of hiding it)

Not quite sure whether the shortening looks ok, but I kind of like it 😄 You can get messages like:

RUN <<EOF (echo $(whoami) >> cmd1)

or

RUN <<EOF (#!/usr/bin/env python3...)

[thaJeztah]

If this code is used to print progress, is it possible to make it "not truncate" when using --progress=plain ? I recall #1435 (also a quick look there, I see it limits at 72 chars, but not sure what's used elsewhere)

[thaJeztah]

^^ discussing with @tonistiigi what options we have. My "case" was that when using --progress=plain (e.g. in CI, or for debugging purposes), as a user I'd want to see the full output, in case there's an issue somewhere half-way my heredoc.

The debate is if we should do so, because heredocs (quite likely) can be huge if people stuff a lot of code in them. 😓

[tonistiigi]

We don't necessarily need to trim the first line. Client-side should take care of that(hopefully it doesn't hide command that is more important than heredoc). But I don't think we should start to convert the doc to something else. Yes, you can't see the full contents and I think that is expected, shell does not print out the contents of files you execute either. User can always set set -x if they want to see internal commands.

[thaJeztah]

We don't necessarily need to trim the first line.

Yes, perhaps that's an ok balance for now 👍

[tonistiigi]

We could also make sure that the line is not empty and pick the next one if it is.

[tonistiigi]

My takeaway from this was that we should still show the first (non-empty) line only(in addition to the command). But we should not do any whitespace trimming on that line. Current changes look like the opposite.

[jedevc]

Ah ok, my apologies, I misunderstood - will fix that up later.

[jedevc]

Have taken a look, now this should just take only the first non-empty line (with no capping) as well as the command.

[tonistiigi]

Where is the non-empty check?

[jedevc]

By doing the TrimSpace above, that would remove all empty lines above - although it would also strip any whitespace at the beginning of the line. Is that something we want to keep as well?

[thaJeztah]

I don't know how to test this, but LGTM (from looking at the code). Thanks!