ci: use the container tag instead of docker, for the build steps

This way, this will use either the docker or the podman runners.

The other steps still use only docker, because I didn’t solve the cache
issue on podman yet.

.gitlab-ci.yml

@@ -36,7 +36,7 @@ variables:
   extends: .sources_changed
   stage: build
   tags:
-    - docker
+    - container
   script:
     - "echo Running cmake with the following parameters: -DCMAKE_CXX_COMPILER=${COMPILER} -DCMAKE_BUILD_TYPE=${BUILD_TYPE} ${BOTAN} ${UDNS} ${SYSTEMD} ${LIBIDN} ${SQLITE3} ${POSTGRESQL}"
     - mkdir build/