allow space before heredoc name

[kasperk81]

in bash, this is allowed:

cat <<       EOF      > /tmp/afile       
  stuff
  more stuff
EOF

Dockerfile are quite pedantic about no space between << and name EOF, otherwise it gives a very confusing syntax error on next line which leads user to think if heredocs are supported, then the follow up google search makes things more confusing... looks to be a simple case of a missing \s* in heredoc parser.

[thaJeztah]

I know the original implementation (see #2132, moby/moby#34423) was based on https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_07_04 - we should probably read up in that document to see how it defines handling of whitespace here.

If the spec doesn't define whitespace being supported, I'm somewhat leaning towards not allowing it (also in light of efforts in the Dockerfile syntax to provide linting errors for consistency), but we should definitely (where possible) improve errors to make it clear to the user what's wrong (i.e., if we decide based on the above to not allow whitespace, then at least a linter and/or error should indicate that it's because of whitespace).

cc @jedevc @tianon

[kasperk81]

yes if the error is improved to say "did you mean <<EOF" that would suffice too (in case allowing space there is not feasible)