Consider removing docker dependency from package.xml

[firesurfer]

I suggest removing

  <test_depend>docker.io</test_depend>

As this is a very large dependency where many people prob. want to explicitly decide whether they install it or not. If implicitly installed it can even be a security risk on some systems especially if users are unaware of it.

There is --skip-keys=SKIP_KEYS for rosdep. But in order to specify this option one has to be aware of what is installed which might not be the case especially for quickly put together testing environments.