parser: do not consume the newline that ends a heredoc terminator line

[Eljees]

closes #99
related-to: hadolint/hadolint#960

What I did

Anything that follows a heredoc immediately — a comment or the next instruction — fails to parse:

RUN <<EOF
echo foo
EOF
# comment

Dockerfile:4:1: unexpected '#'
expecting a new line followed by the next instruction

Both reporters noticed that the error disappears if you add an empty line after the terminator,
or a single space before the #. That is the signature of the defect: the character is fine,
the newline is gone.

How I did it

heredocContent (src/Language/Docker/Parser/Prelude.hs) recognises the terminator as

terEOL = string $ "\n" <> marker <> "\n"

so it consumes the line break that comes after the marker. The top-level loop in
Language/Docker/Parser.hs then asks for a separator:

i <- parseInstruction
eol <|> eof <?> "a new line followed by the next instruction"

and there is nothing left to consume — hence the error, and hence the two workarounds: an empty
line supplies a second newline, and a space before # is caught by the onlySpaces1 branch
of eol.

The patch stops at the marker and only looks ahead for the newline, so the line break stays
available for the caller. delEOL (the empty-heredoc delimiter) has the same shape and the same
defect, and is fixed symmetrically — it is reachable: it is the branch taken by the existing
"empty heredoc" test, which I confirmed with a traceM probe before touching it.

terEOF/delEOF (marker at end of file, no trailing newline) are left alone.

How to verify it

cabal test --test-show-details=always

• five new tests in test/Language/Docker/ParseRunSpec.hs and
  test/Language/Docker/ParseCopySpec.hs: heredoc followed by a comment, heredoc followed by
  another instruction, and the same two for an empty heredoc;
• before the change: 302 examples, 5 failures, each with the exact error from the issues;
• after: 302 examples, 0 failures (the 297 pre-existing tests are untouched), including
  "empty heredoc", "empty heredoc not followed by newline" and "evil heredoc";
• hlint src/ test/ reports the same 5 hints before and after the patch.

Environment: GHC 9.8.4, cabal test as in .github/workflows/haskell.yml.

[m-ildefons]

Thanks a lot. This is awesome.