Heredoc support in parser

[ekzhang]

Hello, noticed this parser doesn't have support for the "heredoc" syntax in dockerfiles, which was added to BuildKit in 2022. This is a pretty recent addition to the Dockerfile syntax.

RUN <<EOF
echo "Hello" >> /hello
echo "World!" >> /hello
EOF

Here is the doc for it: https://github.com/moby/moby/issues/34423

We at Modal can look at helping add support for this!