fix(action): Stop rootful Docker daemon

GitHub Actions started managing the Docker daemon via systemd
unannounced. That caused this action to fail when trying to start the
rootless Docker daemon, because the rootful Docker daemon is already
running. Stop the rootful Docker daemon before attempting to start the
rootless Docker daemon. When Docker is managed via systemd, there is no
need to run dockerd-rootless.sh separately since the rootless Docker
install script starts the rootless Docker daemon automatically.

action.yaml

@@ -24,40 +24,43 @@ runs:
         echo "::set-output name=installed::$installed"
         echo "::set-output name=in-use::$in_use"
       shell: bash
-    - name: Install rootless Docker.
-      if: steps.rootless-docker.outputs.installed != 'true'
-      run: >
-        curl
-        --fail
-        --silent
-        --show-error
-        --location https://get.docker.com/rootless |
-        sh
-      env:
-        FORCE_ROOTLESS_INSTALL: "1"
-      shell: bash
-    - name: Use rootless Docker.
+    - name: Stop Docker daemon.
       if: steps.rootless-docker.outputs.in-use != 'true'
+      run: sudo systemctl stop docker.service docker.socket
+      shell: bash
+    - name: Set environment variables recommended by rootless Docker install script.
+      if: steps.rootless-docker.outputs.installed != 'true'
       run: |
         if [[ -z $XDG_RUNTIME_DIR ]]; then
           echo XDG_RUNTIME_DIR=~/.docker/run >>"$GITHUB_ENV"
         fi
         echo ~/bin >>"$GITHUB_PATH"
-        docker context use rootless
       shell: bash
-    - name: Start rootless Docker daemon, and wait until it's listening.
-      if: steps.rootless-docker.outputs.in-use != 'true'
+    - name: Install rootless Docker, start daemon, and wait until it's listening.
+      if: steps.rootless-docker.outputs.installed != 'true'
       run: |
+        success_sentinel="API listen on $XDG_RUNTIME_DIR/docker.sock"
         function awaitDockerd() {
           while IFS= read -r -t 60 line; do
             echo "$line"
-            [[ "$line" = *"API listen on $XDG_RUNTIME_DIR/docker.sock"* ]] &&
-              return
+            [[ "$line" = *"$success_sentinel"* ]] && return
           done
           echo "Timed out waiting for Docker daemon to listen." >&2
           return 1
         }
 
-        (PATH="/sbin:/usr/sbin:$PATH" dockerd-rootless.sh &) |&
-        awaitDockerd
+        install_script_output="$(curl \
+          --fail \
+          --silent \
+          --show-error \
+          --location https://get.docker.com/rootless |
+          sh)"
+        echo "$install_script_output"
+        docker context use rootless
+        if !grep --quiet "$success_sentinel" <<<"$install_script_output"; then
+          (PATH="/sbin:/usr/sbin:$PATH" dockerd-rootless.sh &) |&
+          awaitDockerd
+        fi
+      env:
+        FORCE_ROOTLESS_INSTALL: "1"
       shell: bash