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.
It's been replaced by cargo-deny in all modules.
Don't regenerate images for now, it's not a problem to keep it around
until the next images update round.
CARGO_HOME's semantic depends on when it's used in the pipeline:
- it's the install prefix when installing Cargo
- it's the crate cache location when building jobs
env.sh is sourced at the start of all jobs and was overidding the
CARGO_HOME path defined in the CI template for caching. Fix this by
moving the prefix install path to install-rust.sh.
