Fix: Add container running check after service restarts

[swetha1654]

Applicable spec:

Overview

Fixes #30

Rationale

Root cause: _wait_for(partial(systemd.service_running, TMATE_SERVICE_NAME), timeout=60 * 10) returns True for a short period of time after the service restarts before returning False, leading to the code falsely assuming that the service is running even though it failed.
Added another check to see if the docker container is running, this check is more robust.

Juju Events Changes

Module Changes

Library Changes

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, complex, documentation)
• The docs/changelog.md is updated with user-relevant changes in the format of keep a changelog v1.1.0

[cbartz]

Thank you Swetha! Looks good, but I have comments related to the tests.