cmdeploy: prepare for being able to run commands in docker containers

[missytake]

Based on #659, this is also pulled out of Keonik1#3. It enables us to run some DNS checks in a docker container; it could also be part of #659, but I put it in a separate PR, as it isn't useful before #614.

[hpk42]

I suggest to rename "docker" to ":docker" throughout the PR to disambiguate from regular ssh hosts.
Also it's a bit weird to merge this PR before the actual docker PR, and there are no tests or CI yet.

[missytake]

What about @docker? pyinfra uses @local for running on localhost, we support that as well, that would make it a bit more consistent.

I wonder if any of those options could mess with SSH configs...

[hpk42]

yes, @docker is better if we have @Local already. i think "ssh @docker" is invalid, so i guess it's ok wrt ssh config. anything with an @ prefix is not an ssh target.

[missytake]

there are no tests or CI yet.

The only thing I see that makes sense to test for this PR is whether pre_command is passed correctly to sub-functions. But I fail mocking the shell() function to see the output of a command instead of executing it. Do you have an idea what I'm doing wrong here? Replacing cmdeploy.remote.rshell.shell with test_cmdeploy.test_dns_when_ssh_docker.<locals>.shell seems to work in all methods except the one that counts.

[hpk42]

roughly looks good (didn't test it for real) but i recommend to consider the LocalExec/DockerExec suggestion.