ci: run tests in Docker (#5440)

This PR adds Chromium with webdriver to the Docker image, so it's now possible to run at least some subset of tests using Docker env.
Additionally, Alpine version has been bumped up, so recent version of Chromium can be used.

build/docker/Dockerfile

@@ -2,14 +2,17 @@
 #   docker build -t shaka-player-build /path/to/shaka-player/build/docker
 # Run with:
 #   docker run -v /path/to/shaka-player:/usr/src --user $(id -u):$(id -g) shaka-player-build
+# Run tests with:
+#   docker run -v /path/to/shaka-player:/usr/src --user $(id -u):$(id -g) shaka-player-build python3 build/test.py --quick --browsers ChromeHeadless
 
-FROM alpine:3.14
+FROM alpine:3.18
 
 # Install dependencies
 RUN apk add --update --no-cache \
-  bash git nodejs npm openjdk11-jre-headless python3
+  bash chromium chromium-chromedriver git nodejs npm openjdk11-jre-headless python3
 
 WORKDIR /usr/src
 ENV HOME /tmp
+ENV CHROMEDRIVER_PATH /usr/bin/chromedriver
 
 CMD ["python3", "build/all.py"]