feat(builder): remove Dockerfile shim

Instead of using a shim Dockerfile, use ONBUILD directives in
slugrunner.

Author: technosophos

builder/rootfs/etc/confd/templates/builder

@@ -118,8 +118,7 @@ if [ ! -f Dockerfile ]; then
 
     # copy out the compiled slug
     docker cp $JOB:/tmp/slug.tgz $TMP_DIR
-    # copy over the Dockerfile shim to the build dir
-    cp $DOCKERFILE_SHIM ./Dockerfile
+    echo "FROM deis/slugrunner" > ./Dockerfile
 fi
 
 # force newline

builder/rootfs/usr/local/share/shim.dockerfile

@@ -18,3 +18,7 @@ ENV HOME /app
 ENTRYPOINT ["/runner/init"]
 
 ENV DEIS_RELEASE 1.10.0-dev
+
+ONBUILD RUN mkdir -p /app
+ONBUILD WORKDIR /app
+ONBUILD ADD slug.tgz /app