Compile dependencies separately

So that they can be more aggressively cached by Docker!  The Docker hub
will take advantage of this, too, which should make our autobuilds
faster.

Author: dcreager

Dockerfile

@@ -1,12 +1,26 @@
 FROM haskell:8.6 as build
 WORKDIR /build
 RUN cabal new-update
+
+# Build our upstream dependencies after copying in only enough to tell cabal
+# what they are.  This will make these layers cache better even as we change the
+# code of semantic itself.
+COPY semantic.cabal /build/
+COPY cabal.project /build/
+COPY semantic-core/semantic-core.cabal /build/semantic-core/
+COPY vendor /build/vendor
+RUN cabal new-build --only-dependencies
+
+# Once the dependencies are built, copy in the rest of the code and compile
+# semantic itself.
 COPY . /build
 RUN cabal new-build semantic:exe:semantic
 
 # A fake `install` target until we can get `cabal new-install` to work
-RUN cp $(find dist-newstyle -name semantic -type f -perm -u=x) /semantic
+RUN cp $(find dist-newstyle -name semantic -type f -perm -u=x) /usr/local/bin/semantic
 
+# Create a fresh image containing only the compiled CLI program, so that the
+# image isn't bulked up by all of the extra build state.
 FROM debian:stretch
 RUN apt-get update && \
   apt-get install -y \
@@ -15,5 +29,5 @@ RUN apt-get update && \
   apt-get autoremove -y && \
   apt-get clean -y && \
   rm -rf /var/lib/apt/lists/*
-COPY --from=build /semantic /usr/local/bin/semantic
+COPY --from=build /usr/local/bin/semantic /usr/local/bin/semantic
 ENTRYPOINT ["/usr/local/bin/semantic"]