Add GitHub Actions CI rules (#70)

* Prepare Dockerfile for GitHub Actions

* Add Action

* Add CI rules

* Move docker files into the docker directory to prevent fetching build artifacts

* Cache Docker layers

Co-authored-by: aanper <mail@s3f.ru>

docker/entrypoint.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+
+# A hack for GitHub Actions to not install Rust twice
+if [ "$HOME" != "/root" ]; then
+    ln -sf /root/.rustup "$HOME/.rustup"
+    ln -sf /root/.cargo "$HOME/.cargo"
+fi
+
+PATH="$HOME/.cargo/bin:${PATH}"
+
+if [ -z "$1" ]; then
+    bash
+else
+    echo "Running $1"
+    set -ex
+    bash -c "$1"
+fi