The GitHub Action workflow uses a cache to speed up the Docker builds.
However, it seems that it only partially works, saving only the Docker
layers for the Firefox build. This is an attempt to solve it by using
two separate cache paths, one for each build.
Add layer caching to speed up the CI builds. In addition, optimize the
Dockerfiles by moving all the dependency installations (which are
unlikely to change much) to the initial layers.
The tests container copies files from the build containers. In order for
this to work in the GitHub actions workflow, the build images need to be
uploaded to a local Docker registry first.
lwthiker