Switch to GHA, GHCR and add the linux-micro image

[pietroalbini]

This PR makes multiple changes to this repository, hopefully improving the experience of using and contributing to crates-build-env:

• The CI provider has been switched from Travis CI to GitHub Actions, as Travis CI occasionally fails to build PRs. The only notable difference is that container builds now start only after the linting process finishes, to avoid building containers when the packages.txt is not properly formatted.
• The hosting for the container images has been switched from Docker Hub over to GitHub Container Registry. Docker Hub recently implemented fairly strict rate limits for unauthenticated downloads, and GHCR is nicely integrated with GitHub permissions and GitHub Actions. The image on Docker Hub will remain available but will not be updated anymore.
• A new container image was added: linux-micro. The container image just installs build-essentials and libssl-dev (OpenSSL), and it's meant to be used for local development and test suites. This should make the experience of contributing to Crater and docs.rs less painful.
• The README was updated to reflect these changes.

After this PR is merged, Rustwide will need to be updated to point to GHCR, and tools will need to download the Docker images with these commands:

docker pull ghcr.io/rust-lang/crates-build-env/linux:latest
docker pull ghcr.io/rust-lang/crates-build-env/linux-micro:latest

r? @jyn514

[pietroalbini]

Addressed all feedback @jyn514!

[jyn514]

r=me with that last nit addressed :)