containers: Drop obsolete/broken docker support

Commit b746f5a dropped support for the `$docker` env variable
in the `start` script.  We don't need it for now, as tests run fine with
podman on the GitHub runners. So drop the setting of the variable, and
always use podman in the build script and refresh workflow.

.github/workflows/unit-tests-refresh.yml

@@ -13,9 +13,6 @@ jobs:
       contents: read
       packages: write
     timeout-minutes: 180
-    env:
-      # current podman backport falls over as non-root, and completely breaks down with :i386 image
-      docker: docker
     steps:
       - name: Clone repository
         uses: actions/checkout@v2
@@ -39,9 +36,9 @@ jobs:
         run: containers/unit-tests/start :i386 distcheck
 
       - name: Log into container registry
-        run: docker login -u ${{ github.actor }} -p ${{ secrets.GITHUB_TOKEN }} ghcr.io
+        run: podman login -u ${{ github.actor }} -p ${{ secrets.GITHUB_TOKEN }} ghcr.io
 
       - name: Push containers to registry
         run: |
-          docker push ghcr.io/cockpit-project/unit-tests:latest
-          docker push ghcr.io/cockpit-project/unit-tests:i386
+          podman push ghcr.io/cockpit-project/unit-tests:latest
+          podman push ghcr.io/cockpit-project/unit-tests:i386

.github/workflows/unit-tests.yml

@@ -12,9 +12,6 @@ jobs:
           - { make: 'distcheck', cc: 'gcc', tag: 'i386' }
       fail-fast: false
     timeout-minutes: 60
-    env:
-      # current podman backport falls over as non-root, and completely breaks down with :i386 image
-      docker: docker
     steps:
       - name: Clone repository
         uses: actions/checkout@v2

containers/unit-tests/build

@@ -3,15 +3,10 @@ set -eu
 
 dir=$(dirname "$0")
 
-if test -z "${docker:=$(which podman || which docker || true)}"; then
-  echo 'Neither podman nor docker are installed'
-  exit 1
-fi
-
 if [ -z "${1:-}" ] || [ "${1:-}" = amd64 ]; then
-    $docker build --build-arg debian_arch=amd64 --build-arg personality=linux64 -t ghcr.io/cockpit-project/unit-tests ${dir}
+    podman build --build-arg debian_arch=amd64 --build-arg personality=linux64 -t ghcr.io/cockpit-project/unit-tests ${dir}
 fi
 
 if [ -z "${1:-}" ] || [ "${1:-}" = i386 ]; then
-    $docker build --build-arg debian_arch=i386 --build-arg personality=linux32 -t ghcr.io/cockpit-project/unit-tests:i386 ${dir}
+    podman build --build-arg debian_arch=i386 --build-arg personality=linux32 -t ghcr.io/cockpit-project/unit-tests:i386 ${dir}
 fi