Fixing CI

[JimTools]

There's an issue with the downstream PPA "ondrej/php" where is cannot find the gearman package.

Refactoring Dockerfile to use standard base images as a workaround

[JimTools]

I've managed to get the majority of the tests to pass. The one which I can't work out is:

TypeError: is_dir(): Argument #1 ($filename) must be of type string, int given

This seems deep in Syfoney when it caches the config in /tmp and makes a proxy call.

Notable changes

With the Dockerfile changes, it may be beneficial to cache the build steps or push the image to a repository for faster build time and less waiting on tests