dockerd-rootless.sh: allow specifying DOCKERD_ROOTLESS_ROOTLESSKIT_PORT_DRIVER

The default port driver "builtin" might not be always preferrable as it
drops src IP information: https://github.com/containers/libpod/pull/6324

Now the port driver can be changed to "slirp4netns" via the environment
variable `DOCKERD_ROOTLESS_ROOTLESSKIT_PORT_DRIVER`.

It is still recommended to use the default "builtin" driver.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

contrib/dockerd-rootless.sh

@@ -7,8 +7,13 @@
 # * newuidmap and newgidmap needs to be installed.
 # * /etc/subuid and /etc/subgid needs to be configured for the current user.
 # * Either one of slirp4netns (>= v0.4.0), VPNKit, lxc-user-nic needs to be installed.
-#   slirp4netns is used by default if installed. Otherwise fallsback to VPNKit.
-#   The default value can be overridden with $DOCKERD_ROOTLESS_ROOTLESSKIT_NET=(slirp4netns|vpnkit|lxc-user-nic)
+#
+# Recognized environment variables:
+# * DOCKERD_ROOTLESS_ROOTLESSKIT_NET=(slirp4netns|vpnkit|lxc-user-nic): the rootlesskit network driver. Defaults to "slirp4netns" if slirp4netns (>= v0.4.0) is installed. Otherwise defaults to "vpnkit".
+# * DOCKERD_ROOTLESS_ROOTLESSKIT_MTU=NUM: the MTU value for the rootlesskit network driver. Defaults to 65520 for slirp4netns, 1500 for other drivers.
+# * DOCKERD_ROOTLESS_ROOTLESSKIT_PORT_DRIVER=(builtin|slirp4netns): the rootlesskit port driver. Defaults to "builtin".
+# * DOCKERD_ROOTLESS_ROOTLESSKIT_SLIRP4NETNS_SANDBOX=(auto|true|false): whether to protect slirp4netns with a dedicated mount namespace. Defaults to "auto".
+# * DOCKERD_ROOTLESS_ROOTLESSKIT_SLIRP4NETNS_SECCOMP=(auto|true|false): whether to protect slirp4netns with seccomp. Defaults to "auto".
 #
 # See the documentation for the further information: https://docs.docker.com/engine/security/rootless/
 
@@ -36,6 +41,7 @@ fi
 
 : "${DOCKERD_ROOTLESS_ROOTLESSKIT_NET:=}"
 : "${DOCKERD_ROOTLESS_ROOTLESSKIT_MTU:=}"
+: "${DOCKERD_ROOTLESS_ROOTLESSKIT_PORT_DRIVER:=builtin}"
 : "${DOCKERD_ROOTLESS_ROOTLESSKIT_SLIRP4NETNS_SANDBOX:=auto}"
 : "${DOCKERD_ROOTLESS_ROOTLESSKIT_SLIRP4NETNS_SECCOMP:=auto}"
 net=$DOCKERD_ROOTLESS_ROOTLESSKIT_NET
@@ -79,7 +85,7 @@ if [ -z $_DOCKERD_ROOTLESS_CHILD ]; then
 		--net=$net --mtu=$mtu \
 		--slirp4netns-sandbox=$DOCKERD_ROOTLESS_ROOTLESSKIT_SLIRP4NETNS_SANDBOX \
 		--slirp4netns-seccomp=$DOCKERD_ROOTLESS_ROOTLESSKIT_SLIRP4NETNS_SECCOMP \
-		--disable-host-loopback --port-driver=builtin \
+		--disable-host-loopback --port-driver=$DOCKERD_ROOTLESS_ROOTLESSKIT_PORT_DRIVER \
 		--copy-up=/etc --copy-up=/run \
 		--propagation=rslave \
 		$DOCKERD_ROOTLESS_ROOTLESSKIT_FLAGS \