Fix MTU configuration for dockerd (#421)

Resolves #393

runner/startup.sh

@@ -17,6 +17,34 @@ function wait_for_process () {
     return 0
 }
 
+sudo /bin/bash <<SCRIPT
+mkdir -p /etc/docker
+
+cat <<EOS > /etc/docker/daemon.json
+{
+EOS
+
+if [ -n "${MTU}" ]; then
+cat <<EOS >> /etc/docker/daemon.json
+  "mtu": ${MTU}
+EOS
+# See https://docs.docker.com/engine/security/rootless/
+echo "environment=DOCKERD_ROOTLESS_ROOTLESSKIT_MTU=${MTU}" >> /etc/supervisor/conf.d/dockerd.conf
+fi
+
+cat <<EOS >> /etc/docker/daemon.json
+}
+EOS
+SCRIPT
+
+INFO "Using /etc/docker/daemon.json with the following content"
+
+cat /etc/docker/daemon.json
+
+INFO "Using /etc/supervisor/conf.d/dockerd.conf with the following content"
+
+cat /etc/supervisor/conf.d/dockerd.conf
+
 INFO "Starting supervisor"
 sudo /usr/bin/supervisord -n >> /dev/null 2>&1 &
 
@@ -27,6 +55,8 @@ for process in "${processes[@]}"; do
     wait_for_process "$process"
     if [ $? -ne 0 ]; then
         ERROR "$process is not running after max time"
+        ERROR "Dumping /var/log/dockerd.err.log to help investigation"
+        cat /var/log/dockerd.err.log
         exit 1
     else 
         INFO "$process is running"