Fix Docker warnings

[gchqdev227]

Fixes #32.

Although the tests were technically running fine on all architectures, the use of an old Mock resource image led to Docker emitting warnings about mismatched architectures, which cluttered up the expected debugging output and failed tests on architectures that didn't match. This PR:

• Upgrades the mock resource image (and specifies a stronger tag)
• Makes this image tag a global variable that CI will extract from the Python to avoid them falling out of sync in future.
• Updated the tests to account for new environment variables present in the new resource container.