Hack to force a fetch of opam repos on every image build

Also, remove old workaround for /tmp bug

Dockerfile

@@ -34,5 +34,6 @@ USER ocaml
 WORKDIR /home/ocaml
 ENV OPAMYES 1
 
-RUN touch /tmp/foo
+ARG LAST_REPO_UPDATE=0
+RUN echo "LAST_REPO_UPDATE=${LAST_REPO_UPDATE}"
 RUN ["/usr/local/bin/opam", "init", "-a", "--bare", "--disable-sandboxing"]

ocaml-image.yml.in

@@ -19,6 +19,7 @@ ocaml-image-%OCAML_VERSION%:
         --context=$CI_PROJECT_DIR
         --dockerfile=$CI_PROJECT_DIR/Dockerfile
         --build-arg=OCAML_VERSION=$OCAML_VERSION
+        --build-arg=LAST_REPO_UPDATE=$(date +%s)
         --destination=$CI_REGISTRY_IMAGE:${OCAML_VERSION%.*}
         $(if [ "$OCAML_VERSION" = "$LATEST_OCAML_VERSION" ]; then
             echo "--destination=$CI_REGISTRY_IMAGE:latest";