ci: switch build to GitHub Actions

ci/docker/README.md

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+These are Docker images used for cross compilation in CI builds (or locally)
+via the [Cross](https://github.com/rust-embedded/cross) tool.
+
+The Cross tool actually provides its own Docker images, and all Docker images
+in this directory are derived from one of them. We provide our own in order
+to customize the environment. For example, we need to install some things like
+`asciidoc` in order to generate man pages. We also install compression tools
+like `xz` so that tests for the `-z/--search-zip` flag are run.
+
+If you make a change to a Docker image, then you can re-build it. `cd` into the
+directory containing the `Dockerfile` and run:
+
+    $ cd x86_64-unknown-linux-musl
+    $ ./build
+
+At this point, subsequent uses of `cross` will now use your built image since
+Docker prefers local images over remote images. In order to make these changes
+stick, they need to be pushed to Docker Hub:
+
+    $ docker push burntsushi/cross:x86_64-unknown-linux-musl
+
+Of course, only I (BurntSushi) can push to that location. To make `cross` use
+a different location, then edit `Cross.toml` in the root of this repo to use
+a different image name for the desired target.