Fixed building images with different dockerfile

added missing argument in build hook file for building with different dockerfille

Author: SloCompTech

hooks/build

@@ -13,4 +13,4 @@ docker build \
   --build-arg VCS_REF=`git rev-parse --short HEAD` \
   --build-arg VCS_SRC="https://github.com/SloCompTech/docker-openvpn/commit/$SOURCE_COMMIT" \
   --build-arg VERSION="$SOURCE_BRANCH" \
-  -t $IMAGE_NAME .
+  -t $IMAGE_NAME -f $DOCKERFILE_PATH .