frontend/dockerfile: Chomp heredoc before parsing shebang

[MTCoster]

Fixes #5383.

Tested locally using the following MCRE:

FROM debian:stable

RUN <<EOF
#!/usr/bin/bash
declare -r FOO=foo
EOF

RUN <<-EOF
#!/usr/bin/bash
	declare -r BAR=bar
EOF

RUN <<-EOF
	#!/usr/bin/bash
	declare -r BAZ=baz
EOF

Without this change, the final RUN causes building to fail with /bin/sh: 2: declare: not found since the shebang is not detected.

[jedevc]

LGTM 🎉

Can you please add a test case to testRunShebangHeredoc in https://github.com/moby/buildkit/blob/master/frontend/dockerfile/dockerfile_heredoc_test.go#291 before we merge?

[jedevc]

Can you write it to a different file, and update the assertions below as well.

[MTCoster]

That's what I get for rushing 🤦 Should be fixed now.