ci: build base docker image once a week on schedule

ci/environment.yml

@@ -1,7 +1,6 @@
-environment:
+.environment:
   stage: environment
   image: docker
-  when: manual
   variables:
     GIT_SUBMODULE_STRATEGY: none  # no need to fetch submodules
     CONTAINER_NAME: "$CI_REGISTRY/satoshilabs/trezor/trezor-firmware/trezor-firmware-env.nix"
@@ -20,3 +19,13 @@ environment:
     - docker build --tag $CONTAINER_NAME:$CI_COMMIT_SHA --tag $CONTAINER_NAME:latest --build-arg ALPINE_VERSION="$ALPINE_VERSION" --build-arg ALPINE_ARCH="$ALPINE_ARCH" --build-arg NIX_VERSION="$NIX_VERSION" --build-arg FULLDEPS_TESTING=1 ci/
     - docker push $CONTAINER_NAME:$CI_COMMIT_SHA
     - docker push $CONTAINER_NAME:latest
+
+environment manual:
+  extends: .environment
+  when: manual
+
+environment scheduled:
+  extends: .environment
+  only:
+    variables:
+      - $SCHEDULED_ENVIRONMENR_BUILD == "true"