Fix conflict on heredoc temp files

[kmichel-aiven]

What type of PR is this?

/kind bug

What this PR does / why we need it:

If two image builds start concurrently and they both use an heredoc with the same EOF marker, then they will use the same filename and overwrite each other.

It's possible for concurrent builds to either use the wrong heredoc content or to fail because the file of one build is deleted by the other, leading to this error message:

STEP 13/21: COPY <<EOF /file.conf

Error: building at STEP "COPY <<EOF /file.conf": reading "/var/tmp/EOF": copier: get: globs [/EOF] matched nothing (0 filtered out): no such file or directory

How to verify it

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?