fixes for docker.yml + toolchain.dockerfile

[tracedgod]

fixes for docker.yml + toolchain.dockerfile

This pull request features changes to the Github Actions docker runner job as well as the libxenon.dockerfile build. My reason for this PR is I had noticed the Free60 Docker Hub images appear to be quite behind from what the Github repo has.

Github Actions

• Fixes deprecation warnings encountered by updating the tags for the respective actions

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Dockerfile(s)

• Fixes failing build steps to the docker images by upgrading the base image from ubuntu:21.04 to ubuntu:22.04
• Combine both the toolchain.dockerfile & the libxenon.dockerfile builds into a single libxenon docker image build by utilizing a multi-stage dockerfile solution (this removes the toolchain.dockerfile and instead runs those steps as the base image in the libxenon.dockerfile build)

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Also a question: Is there a specific reason the toolchain and libxenon images are separate other than the ability to use the free60/toolchain image once it's built in the next stage for building free60/libxenon?