Add some more specific help for Dockerfile build contexts.

I've copy and pasted the error message so its googlable.

Docker-DCO-1.1-Signed-off-by: SvenDowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Merge pull request #5762 from SvenDowideit/pr_out_add_some_more_specific_help_for_dockerfile_build_contexts

Add some more specific help for Dockerfile build contexts.