tox.ini (docker): Supply BUILD_TAG to write-dockerfile.sh, for use by…

… the default from_docker_tag in docker.yml

tox.ini

@@ -765,7 +765,7 @@ commands =
     local:         bash -c 'if [ ! -d prefix -o -L prefix ]; then rm -f prefix; ln -sf {env:PREFIX:{envdir}/local} prefix; fi'
 
 ##commands =
-    docker:        bash -c '.ci/write-dockerfile.sh {env:SYSTEM} "{env:SAGE_PACKAGE_LIST_ARGS:}" {env:WITH_SYSTEM_SPKG} {env:IGNORE_MISSING_SYSTEM_PACKAGES} "{env:ALL_EXTRA_SAGE_PACKAGES}" > {envdir}/Dockerfile'
+    docker:        bash -c 'BUILD_TAG={env:DOCKER_TAG:$(git describe --dirty --always)} .ci/write-dockerfile.sh {env:SYSTEM} "{env:SAGE_PACKAGE_LIST_ARGS:}" {env:WITH_SYSTEM_SPKG} {env:IGNORE_MISSING_SYSTEM_PACKAGES} "{env:ALL_EXTRA_SAGE_PACKAGES}" > {envdir}/Dockerfile'
     # From https://hub.docker.com/r/multiarch/ubuntu-core/
     # configure binfmt-support on the Docker host (works locally or remotely, i.e: using boot2docker)
     docker-{arm64,armhf}:  docker run --rm --privileged multiarch/qemu-user-static:register --reset