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.

ci/docker/arm-unknown-linux-gnueabihf/Dockerfile

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+FROM rustembedded/cross:arm-unknown-linux-gnueabihf
+
+COPY stage/ubuntu-install-packages /
+RUN /ubuntu-install-packages

ci/docker/arm-unknown-linux-gnueabihf/build

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+#!/bin/sh
+
+mkdir -p stage
+cp ../../ubuntu-install-packages ./stage/
+docker build -t burntsushi/cross:arm-unknown-linux-gnueabihf .

ci/docker/i686-unknown-linux-gnu/Dockerfile

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+FROM rustembedded/cross:i686-unknown-linux-gnu
+
+COPY stage/ubuntu-install-packages /
+RUN /ubuntu-install-packages

ci/docker/i686-unknown-linux-gnu/build

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+#!/bin/sh
+
+mkdir -p stage
+cp ../../ubuntu-install-packages ./stage/
+docker build -t burntsushi/cross:i686-unknown-linux-gnu .

ci/docker/mips64-unknown-linux-gnuabi64/Dockerfile

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+FROM rustembedded/cross:mips64-unknown-linux-gnuabi64
+
+COPY stage/ubuntu-install-packages /
+RUN /ubuntu-install-packages

ci/docker/mips64-unknown-linux-gnuabi64/build

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+#!/bin/sh
+
+mkdir -p stage
+cp ../../ubuntu-install-packages ./stage/
+docker build -t burntsushi/cross:mips64-unknown-linux-gnuabi64 .

ci/docker/x86_64-unknown-linux-musl/Dockerfile

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 FROM rustembedded/cross:x86_64-unknown-linux-musl
 
-RUN apt-get update \
- && apt-get install -y --no-install-recommends \
-      libxslt1-dev asciidoc docbook-xsl xsltproc libxml2-utils
+COPY stage/ubuntu-install-packages /
+RUN /ubuntu-install-packages

ci/docker/x86_64-unknown-linux-musl/build

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+#!/bin/sh
+
+mkdir -p stage
+cp ../../ubuntu-install-packages ./stage/
+docker build -t burntsushi/cross:x86_64-unknown-linux-musl .