Dockerfile - support syntax highlight for heredoc format

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Proposal

Right now GitLab (as of version 15.8) does not support syntax highlight for heredocs in Dockerfiles. And it should :)

For example, we are migrating from the classic format to "here document" and we no longer see syntax highlight for flags (like -y or --no-install-recommends) or shell variables (${}). Please take a look at the following screenshot: