docker image, new caching, parallel execution

[TriplEight]

• The cache you've had previously (CARGO_HOME and CARGO_TARGET_DIR): Cache Size: ~2712 MB (2843730371 B) and 4 min to download it. Now it includes SCCACHE_DIR: Cache Size: ~3041 MB (3188684782 B) and 4.5 min to download it. This is a rather a large size, GitHub only allows 5Gb per repo. It can be helped by not including the built binaries, but only if their size is somewhat significant.
• parallel execution (nightly clippy and stable check and test) is more effective: 8:40 min vs 11:30 in consecutive. And it still fails fast, the CI stops i.e. if clippy fails. The downside is both pipelines are downloading the docker image and install clippy.
• docker image instead of installed toolchain. Docker image is heavy and it takes 50 sec to download it, but it's the same time as CI would install a toolchain plus sccache.

[wheresaddie]

since @JoshOrndorff is no longer active on the project officially, I might suggest deferring to @jimmychu0807 or @riusricardo for the future. Thanks for all your PRs @TriplEight