docker/windows: create a rust image for gst-rs

Create an image on top of the existing windows image, where it has a version of gstreamer per-installed as well as a rust toolchain so it can be used for the gst-rs CI.

See gstreamer-rs!701 (merged)

Based on the build image for the existing windows jobs,
add an image with rust toolchain and a (stable) gstreamer
install for the bindings to build against.

See https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/701

docker/windows: build rust image in the ci

This makes it a bit harder to build an image locally with the script,
as now the rust image requires to pass it the base image tag as a
--build-arg. This way though we don't need to duplicate and keep
the string of the image in sync.

Another thing is that now the docker windows image build is properly
passing/replacing build args and we can build non-master versions
of the image too.

docker/windows: build two version of the rust image

Build 2 images with different rust toolchains so we can test
both the latest release, as well as the minimum supported
compiler version and catch regressions.