dockerfile: fix heredoc parsing with space after modifier

[kasperk81]

Fix this case: #5778 (comment)

[kasperk81]

this works

FROM alpine:3.22
RUN cat <<-EOF > /hello.txt
hello
EOF

RUN cat /hello.txt

this too:

FROM alpine:3.22
RUN cat << EOF > /hello.txt
hello
EOF

RUN cat /hello.txt

but this was failing with dockerfile parse error on line 3: unknown instruction: hello:

FROM alpine:3.22
RUN cat <<- EOF > /hello.txt
hello
EOF

RUN cat /hello.txt

[thaJeztah]

this works

FROM alpine:3.22
RUN cat <<-EOF > /hello.txt
hello
EOF

RUN cat /hello.txt

this too:

FROM alpine:3.22
RUN cat << EOF > /hello.txt
hello
EOF

RUN cat /hello.txt

Both of those are expected to fail, because there's no Dockerfile here-doc used, so they're line-terminated;

This is the Dockerfile RUN instruction;

RUN cat << EOF > /hello.txt

And hello and EOF are parsed as separate Dockerfile instructions, which are invalid

[thaJeztah]

This doesn't look correct (see my comment); it's conflating here-docs used in a shell with here-docs provided by the Dockerfile parser.

[kasperk81]

Both of those are expected to fail, because there's no Dockerfile here-doc used, so they're line-terminated;

they both work, do you mean that's a bug that they aren't failing?