Run workflows in Mono Docker containers

[DasSkelett]

Motivation

The CI builds still take quite a long time to finish, since installing Mono takes around 10 minutes.
@techman83 had the excellent idea of running the builds in Docker containers from the images published here.
Downloading the images should be way faster than installing and pre-compiling it.

Changes

There exists a container keyword for workflow jobs: https://help.github.com/en/actions/reference/workflow-syntax-for-github-actions#jobsjob_idcontainer
If I understood the documentation correctly, it makes all steps of this job run in the specified container after pulling it from the Docker hub.
Let's see if it works.

Also realized that I messed up the Discord notifications in #3089 since the env var is named differently.

[DasSkelett]

Well, that was fast 😮

Added the apt-get update to the wrong line in release.yml though.

[HebaruSan]

🏃

[HebaruSan]

🚴

[DasSkelett]

The workflow is not valid. .github/workflows/deploy.yml (Line: 15, Col: 14): Unrecognized named-value: 'env'. Located at position 1 within expression: env.RELEASE_MONO_VERSION

Almost...

You can only use the env context in the value of the with and name keys, or in a step's if conditional.

[DasSkelett]

Hmm, I could either specify the mono version directly in the image: line, since it's the only place we use it,
or create a matrix with a single item.

[techman83]

This is really neat! That makes the builds way faster and less prone to issues with installing all the mono packages across so many versions.