Add Docker caching to GitHub actions workflow

Add layer caching to speed up the CI builds. In addition, optimize the
Dockerfiles by moving all the dependency installations (which are
unlikely to change much) to the initial layers.

chrome/Dockerfile

@@ -7,6 +7,10 @@ WORKDIR /build
 RUN apt-get update && \
     apt-get install -y git g++ cmake golang-go ninja-build curl unzip zlib1g-dev libbrotli-dev
 
+# The following are needed because we are going to change some autoconf scripts,
+# both for libnghttp2 and curl.
+RUN apt-get install -y autoconf automake autotools-dev pkg-config libtool
+
 # BoringSSL doesn't have versions. Choose a commit that is used in a stable
 # Chromium version.
 ARG BORING_SSL_COMMIT=3a667d10e94186fd503966f5638e134fe9fb4080
@@ -33,10 +37,6 @@ RUN mkdir boringssl/build/lib && \
 ARG NGHTTP2_VERSION=nghttp2-1.46.0
 ARG NGHTTP2_URL=https://github.com/nghttp2/nghttp2/releases/download/v1.46.0/nghttp2-1.46.0.tar.bz2
 
-# The following are needed because we are going to change some autoconf scripts,
-# both for libnghttp2 and curl.
-RUN apt-get install -y autoconf automake autotools-dev pkg-config libtool
-
 # Download nghttp2 for HTTP/2.0 support.
 RUN curl -o ${NGHTTP2_VERSION}.tar.bz2 -L ${NGHTTP2_URL}
 RUN tar xf ${NGHTTP2_VERSION}.tar.bz2