Clear warning messages when docker is not running or installed

[emersonknapp]

Description

In this issue the user experienced confusion when encountering a problem with docker. It is unclear whether it was not installed, the user was not in the correct group, or something else.

Add a detection step at startup to find out whether docker is installed, running, and accessible, and print clear warning messages about any bad situations detected.

Related Issues

Related to #201

Completion Criteria

• Print a warning message and exit when Docker is not running/installed on the system
• Print a warning message and exit when the user does not have permission to run docker containers

Implementation Notes / Suggestions

• For presence, check for the docker sock file?
• For permission, try running hello-world and catch exception?

Testing Notes / Suggestions

This may be difficult to test, as it requires a specific test environment with the docker dependency missing. We could mock the checked situations at least to test the warning prints.