Dockerfile heredocs in other RUN-like commands

Following up on #2132.

Heredocs are now supported in RUN commands - it might be nice to extend this support into other commands that use similar semantics:

For example, CMD:

CMD <<EOF
#!/usr/bin/env python
print("Hello world")
EOF

Similarly for ONBUILD:

ONBUILD RUN <<EOF
echo test
EOF
ONBUILD COPY <<EOF /dest
file contents
EOF

I also imagine the same would work well with HEALTHCHECK? Though I don't have much experience with that specific command.

This is mostly for the sake of consistency with RUN-like commands, but it's not too hard to imagine valid use cases that might call for this feature.