daemon: add --userland-proxy-path flag

Signed-off-by: Antonio Murdaca <runcom@redhat.com>

docs/reference/commandline/dockerd.md

@@ -78,6 +78,7 @@ Options:
       --tlskey=~/.docker/key.pem             Path to TLS key file
       --tlsverify                            Use TLS and verify the remote
       --userland-proxy=true                  Use userland proxy for loopback traffic
+      --userland-proxy-path=""               Path to the userland proxy binary
       --userns-remap                         User/Group setting for user namespaces
       -v, --version                          Print version information and quit
 ```
@@ -1149,6 +1150,7 @@ This is a full example of the allowed configuration options on Linux:
 	"ip-forward": false,
 	"ip-masq": false,
 	"userland-proxy": false,
+	"userland-proxy-path": "/usr/libexec/docker-proxy",
 	"ip": "0.0.0.0",
 	"bridge": "",
 	"bip": "",

man/dockerd.8.md

@@ -64,6 +64,7 @@ dockerd - Enable daemon mode
 [**--tlskey**[=*~/.docker/key.pem*]]
 [**--tlsverify**]
 [**--userland-proxy**[=*true*]]
+[**--userland-proxy-path**[=*""*]]
 [**--userns-remap**[=*default*]]
 
 # DESCRIPTION
@@ -272,6 +273,9 @@ output otherwise.
 **--userland-proxy**=*true*|*false*
     Rely on a userland proxy implementation for inter-container and outside-to-container loopback communications. Default is true.
 
+**--userland-proxy-path**=""
+  Path to the userland proxy binary.
+
 **--userns-remap**=*default*|*uid:gid*|*user:group*|*user*|*uid*
     Enable user namespaces for containers on the daemon. Specifying "default" will cause a new user and group to be created to handle UID and GID range remapping for the user namespace mappings used for contained processes. Specifying a user (or uid) and optionally a group (or gid) will cause the daemon to lookup the user and group's subordinate ID ranges for use as the user namespace mappings for contained processes.