Shellcheck: Dockerfile support

[bbarker]

For new checks and feature suggestions

• [x] shellcheck.net (i.e. the latest commit) currently gives no useful warnings about this
• [x] I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

Dockerfiles seem to support a limited subset of shell scripts (and shell scripts can be embedded, which is currently quite nasty). It might be good to add this feature at some pont.

[Dmole]

Dockerfiles are not shell files they are config files for which there are already a few validators. Mixing languages is a bug; all shell scripts in dockerfiles should be called with

RUN ex.sh

[kbulygin]

@Dmole Would you call Dockerfiles and Makefiles actually used in many projects bugs too? I think the issue is not about what and who should use.

I would also like to lint embedded scripts (but it's not trivial and quite specific, so I doubt it will be implemented in shellcheck itself). A workaround for such cases is to use a wrapper than conditionally turn files into temporary scripts before linting them. As an illustration (really quick and dirty):

• for Dockerfiles: shellcheck -s bash <(sed 's/^RUN //;' Dockerfile) (or even just shellcheck Dockerfile)
• for Makefiles: shellcheck -s bash <(perl -pe 's/^(?!\t).*//' Makefile) (the substitution is to preserve line numbers)

[koalaman]

You'll love hadolint!

It's a linting tool for Dockerfiles, and it uses ShellCheck for RUN statements.