fix(builder): explicitly call out $TMP_DIR on docker cp

moby/moby@31d1d73
prevents a breakout in ApplyLayer, which also seems to break
`docker cp <container_id>:<src_path> .`. This change explicitly calls
out our working directory (TMP_DIR) which behaves the same as before.

Author: Matthew Fisher

builder/templates/builder

@@ -118,7 +118,7 @@ if [ ! -f Dockerfile ]; then
     docker attach $JOB
 
     # copy out the compiled slug
-    docker cp $JOB:/tmp/slug.tgz .
+    docker cp $JOB:/tmp/slug.tgz $TMP_DIR
     # copy over the Dockerfile shim to the build dir
     cp $DOCKERFILE_SHIM ./Dockerfile
 fi