Dockerfile: allow Docker to cache build steps (#1336)

- By having a `COPY . xx` step before installing dependencies, we rob
Docker of the opportunity to cache downloading and building dependencies
if we make any changes to any files in the repository. By copying only
Gemfiles before the build step, we only require Docker to rebuild
dependencies when the Gemfiles have changed.

Co-authored-by: Matthew Peveler <matt.peveler@gmail.com>

Dockerfile

@@ -7,7 +7,8 @@ VOLUME /srv/slate/source
 
 EXPOSE 4567
 
-COPY . /srv/slate
+COPY Gemfile .
+COPY Gemfile.lock .
 
 RUN apt-get update \
     && apt-get install -y --no-install-recommends \
@@ -17,8 +18,11 @@ RUN apt-get update \
     && bundle install \
     && apt-get remove -y build-essential \
     && apt-get autoremove -y \
-    && rm -rf /var/lib/apt/lists/* \
-    && chmod +x /srv/slate/slate.sh
+    && rm -rf /var/lib/apt/lists/*
+
+COPY . /srv/slate
+
+RUN chmod +x /srv/slate/slate.sh
 
 ENTRYPOINT ["/srv/slate/slate.sh"]
 CMD ["build"]