Allow more Docker build caching

Dockerfile

@@ -1,23 +1,34 @@
-FROM ubuntu:16.04 AS build
-
-# Install GHC and cabal
+# Build-only image
+FROM ubuntu:17.10 AS build
 USER root
 WORKDIR /opt/shellCheck
 
-COPY . .
+# Install OS deps
+RUN apt-get update && apt-get install -y ghc cabal-install
 
-RUN apt-get update && apt-get install -y \
-  ghc \
-  cabal-install
-RUN cabal update && \
-  cabal install --dependencies-only
+# Install Haskell deps
+# (This is a separate copy/run so that source changes don't require rebuilding)
+COPY ShellCheck.cabal ./
+RUN cabal update && cabal install --dependencies-only
+
+# Copy source and build it
+COPY LICENSE Setup.hs shellcheck.hs ./
+COPY src src
 RUN cabal build Paths_ShellCheck && \
-  ghc -optl-static -optl-pthread -idist/build/autogen --make shellcheck && \
+  ghc -optl-static -optl-pthread -isrc -idist/build/autogen --make shellcheck && \
   strip --strip-all shellcheck
 
 RUN mkdir -p /out/bin && \
   cp shellcheck  /out/bin/
 
+# Resulting Alpine image
+FROM alpine:latest
+LABEL maintainer="Vidar Holen <vidar@vidarholen.net>"
+COPY --from=build /out /
+
+# DELETE-MARKER (Remove everything below to keep the alpine image)
+
+# Resulting ShellCheck image
 FROM scratch
 LABEL maintainer="Vidar Holen <vidar@vidarholen.net>"
 WORKDIR /