dockerfile: allow whitespace in heredocs

[jedevc]

Fixes #5778.

FYI @tianon @thaJeztah

[kasperk81]

all these are valid in shells:

# no spaces
cat <<EOF> /tmp/afile

# leading and trailing spaces
cat <<       EOF      > /tmp/afile

# leading space only
cat <<       EOF> /tmp/afile

# trailing space only
cat <<EOF    > /tmp/afile

[jedevc]

Woops, forgot we needed to handle the word splitting as well, should work properly now 👀

All of the above should be valid - we don't need to do any additional word to handle the trailing spaces, those work as intended today.

[thaJeztah]

Thanks!! Looks less complicated than I expected it to be (which is good!).

Found one typo; otherwise just me rambling 😂

[thaJeztah]

No need to change, but this got me thinking; "wasn't there some utility or const in stdlib for these?".

I think having this utility is readable for sure, so definitely no need to swap (just me now getting curious 😂 - I'll probably start digging to find)

[jedevc]

unicode.IsSpace does roughly this, but not quite 😢 it also includes newlines, which we're not interested in.

[thaJeztah]

Heh; I actually didn't know (or forgot) these were allowed as well

[thaJeztah]

Also me thinking out loud; should we (at some point) consider capturing these, so that we could reformat in a canonical form (whitespace removed) 🤔

Also curious now if we have a useful error if we found the start "word", but didn't find the "end" word (i.e., user either forgot, or (e.g..) accidentally indented.

It would be really nice if we could show a usable error for them ("did you mean ... <non-indented> ?")

docker build -<<'EOF'
FROM alpine
RUN <<'EOD'
echo hello
  EOD
RUN echo world
RUN echo that's it
EOF

Currently, it just continues going, and marking the whole remainder of the Dockerfile to be part of the here document.

Dockerfile:2
--------------------
   1 |     FROM alpine
   2 | >>> RUN <<'EOD'
   3 | >>> echo hello
   4 | >>>   EOD
   5 | >>> RUN echo world
   6 | >>> RUN echo that's it
   7 |
--------------------
ERROR: failed to solve: unterminated heredoc

Which is "correct", but possibly could be made smarter (seeing RUN at start of the line could be deducted as; PROBABLY you meant to terminate here, and seeing an indented <word> could be detected as "did you accidentally add whitespace?")

[jedevc]

Ahh, I remember thinking about this, but it kind of ends up involving totally different layers of the lexer/parser to produce this warning, so the effort to produce it seems way higher than the benefit of such a message.

I'm also not entirely convinced that unterminated heredoc is that bad of an error message, especially given that it highlights which line it starts on.