Add MTU propagation docker-shim also to rootless dind runner images

Related to #1201

runner/actions-runner-dind-rootless.dockerfile

@@ -106,6 +106,10 @@ COPY entrypoint.sh logger.bash rootless-startup.sh update-status /usr/bin/
 
 RUN chmod +x /usr/bin/rootless-startup.sh /usr/bin/entrypoint.sh
 
+# Copy the docker shim which propagates the docker MTU to underlying networks
+# to replace the docker binary in the PATH.
+COPY docker-shim.sh /usr/local/bin/docker
+
 # Make the rootless runner directory executable
 RUN mkdir /run/user/1000 \
     && chown runner:runner /run/user/1000 \

runner/actions-runner-dind.dockerfile

@@ -103,6 +103,10 @@ COPY entrypoint.sh logger.bash startup.sh update-status /usr/bin/
 COPY supervisor/ /etc/supervisor/conf.d/
 RUN chmod +x /usr/bin/startup.sh /usr/bin/entrypoint.sh
 
+# Copy the docker shim which propagates the docker MTU to underlying networks
+# to replace the docker binary in the PATH.
+COPY docker-shim.sh /usr/local/bin/docker
+
 # Configure hooks folder structure.
 COPY hooks /etc/arc/hooks/
 

runner/docker-shim.sh

@@ -2,11 +2,16 @@
 
 set -Eeuo pipefail
 
+DOCKER=/usr/bin/docker
+if [ ! -e $DOCKER ]; then
+  DOCKER=$HOME/bin/docker
+fi
+
 if [[ ${ARC_DOCKER_MTU_PROPAGATION:-false} == true ]] &&
   (($# >= 2)) && [[ $1 == network && $2 == create ]] &&
-  mtu=$(/usr/bin/docker network inspect bridge --format '{{index .Options "com.docker.network.driver.mtu"}}' 2>/dev/null); then
+  mtu=$($DOCKER network inspect bridge --format '{{index .Options "com.docker.network.driver.mtu"}}' 2>/dev/null); then
   shift 2
   set -- network create --opt com.docker.network.driver.mtu="$mtu" "$@"
 fi
 
-exec /usr/bin/docker "$@"
+exec $DOCKER "$@"