Fix missing Dockerfile ARG

Docker ARG REPOS_FILE was not redeclared in the "cacher" build stage,
and so was not inheriting the default value specified at the top of the
file. This redeclares the arg in the "cacher" build stage, so that the
default value may be used.

Author: christopherjreid

Dockerfile

@@ -21,6 +21,7 @@ FROM $FROM_IMAGE AS cacher
 
 # clone overlay source
 ARG OVERLAY_WS
+ARG REPOS_FILE
 WORKDIR $OVERLAY_WS/src
 COPY ./${REPOS_FILE} ../
 RUN vcs import ./ < ../${REPOS_FILE} && \