Trac #30933: GH Actions: Repair upload of docker images to docker.pkg.github.com

These docker images are built by the portability CI workflows for Linux.

However, refactoring of the workflow had broken the upload to
`docker.pkg.github.com`. (Although the login to `docker.pkg.github.com`
succeeds, the push fails with `no basic auth credentials` - see
https://github.com/mkoeppe/sage/runs/1414148202?check_suite_focus=true)

URL: https://trac.sagemath.org/30933
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Dima Pasechnik

Author: Release Manager

.github/workflows/tox-experimental.yml

@@ -83,6 +83,7 @@ jobs:
             fi
             if echo "$TOKEN" | docker login docker.pkg.github.com -u ${{ github.actor }} --password-stdin; then
               echo "DOCKER_PUSH_REPOSITORY=docker.pkg.github.com/${{ github.repository }}/" >> $GITHUB_ENV
+              echo "DOCKER_CONFIG_FILE=$HOME/.docker/config.json" >> $GITHUB_ENV
             fi
       - run: |
           set -o pipefail; EXTRA_DOCKER_BUILD_ARGS="--build-arg USE_MAKEFLAGS=\"-k V=0 SAGE_NUM_THREADS=3\"" tox -e $TOX_ENV -- $TARGETS 2>&1 | sed "/^configure: notice:/s|^|::warning file=artifacts/$LOGS_ARTIFACT_NAME/config.log::|;/^configure: warning:/s|^|::warning file=artifacts/$LOGS_ARTIFACT_NAME/config.log::|;/^configure: error:/s|^|::error file=artifacts/$LOGS_ARTIFACT_NAME/config.log::|;"

.github/workflows/tox-optional.yml

@@ -83,6 +83,7 @@ jobs:
             fi
             if echo "$TOKEN" | docker login docker.pkg.github.com -u ${{ github.actor }} --password-stdin; then
               echo "DOCKER_PUSH_REPOSITORY=docker.pkg.github.com/${{ github.repository }}/" >> $GITHUB_ENV
+              echo "DOCKER_CONFIG_FILE=$HOME/.docker/config.json" >> $GITHUB_ENV
             fi
       - run: |
           set -o pipefail; EXTRA_DOCKER_BUILD_ARGS="--build-arg USE_MAKEFLAGS=\"-k V=0 SAGE_NUM_THREADS=3\"" tox -e $TOX_ENV -- $TARGETS 2>&1 | sed "/^configure: notice:/s|^|::warning file=artifacts/$LOGS_ARTIFACT_NAME/config.log::|;/^configure: warning:/s|^|::warning file=artifacts/$LOGS_ARTIFACT_NAME/config.log::|;/^configure: error:/s|^|::error file=artifacts/$LOGS_ARTIFACT_NAME/config.log::|;"

.github/workflows/tox.yml

@@ -76,6 +76,7 @@ jobs:
             fi
             if echo "$TOKEN" | docker login docker.pkg.github.com -u ${{ github.actor }} --password-stdin; then
               echo "DOCKER_PUSH_REPOSITORY=docker.pkg.github.com/${{ github.repository }}/" >> $GITHUB_ENV
+              echo "DOCKER_CONFIG_FILE=$HOME/.docker/config.json" >> $GITHUB_ENV
             fi
       - run: |
           set -o pipefail; EXTRA_DOCKER_BUILD_ARGS="--build-arg USE_MAKEFLAGS=\"-k V=0 SAGE_NUM_THREADS=3\"" tox -e $TOX_ENV -- $TARGETS 2>&1 | sed "/^configure: notice:/s|^|::warning file=artifacts/$LOGS_ARTIFACT_NAME/config.log::|;/^configure: warning:/s|^|::warning file=artifacts/$LOGS_ARTIFACT_NAME/config.log::|;/^configure: error:/s|^|::error file=artifacts/$LOGS_ARTIFACT_NAME/config.log::|;"

tox.ini

@@ -138,6 +138,8 @@ passenv =
     docker:       DOCKER_TARGETS
                   # If set, we use this prefix and push to it
     docker:       DOCKER_PUSH_REPOSITORY
+                  # If set, we symlink this file into {envdir}/.docker/; this can be used for providing credentials for pushing
+    docker:       DOCKER_CONFIG_FILE
     local:        MAKE
     local:        PREFIX
     local:        SAGE_NUM_THREADS
@@ -593,6 +595,7 @@ commands =
     # 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
+    docker:        bash -c 'if [ x"{env:DOCKER_CONFIG_FILE:}" != x ]; then mkdir -p {envdir}/.docker && ln -sf $(realpath "{env:DOCKER_CONFIG_FILE:}") {envdir}/.docker/; fi'
     docker:        bash -c 'for docker_target in {env:DOCKER_TARGETS:with-targets}; do \
     docker:            BUILD_TAG={env:DOCKER_PUSH_REPOSITORY:}sage-{envname}-$docker_target:$(git describe --dirty --always); \
     docker:            DOCKER_BUILDKIT={env:DOCKER_BUILDKIT:0} \