docker: Containerized testsuite and build containers in CI workflow

[rdmark]

Create a Docker container that builds both netatalk and the testsuite. While the baseline spectest test suite can be run remotely, the tier 2 spectest requires access to the local filesystem, so I opted to have the netatalk instance under test in the same container as the testsuite. AFP requests still done over port 548 but on a local socket.

Additionally, create a new GitHub actions workflow that builds the Docker containers, publishes them to the GitHub container store, and then runs the two containerized spectest suites.

[sonarcloud[bot]]

Quality Gate passed

Issues
6 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud