add test where runMetacheck is run inside of runtime container

[maxheld83]

haha, this was a fun one just now solved in https://github.com/subugoe/metacheck/actions/runs/413556533.

There was apparently (?) a missing dependency, which never came up before because:

• it was only needed when there were missing DOIs, which did not come up in much of the testing
• the dependency (gluedown) is probably part of muggle-buildtime anyway (?), so it never came up inside ghactions testing
• inside azure, muggle-runtime probably did not have this, and that's why it failed.

This is a pretty bad failure mode.
Fixes:

• also run (smoke?) test on muggle-runtime in github actions (though even the entrypoint wouldn't have captured that?!)
• don't use any dependencies in the rmarkdown template, because forgetting those doesn't get caught in R CMD check (as per refactor ELT + analysis codebase #52).