At the moment we are building 3 independent images, each one having a different
Rust version. This works perfectly fine but we can save storage space and
bandwith by making a smarter use of Docker's layering system.
Introducing a new 'base' image containing all the deps, including
GStreamer, acting as a base for the actual images.
As a result most of the actual content is now shared accross the same
Docker layer. This would save us from downloading/building all the deps
when updating images and will reduce storage and transfers for runners.
