[proxy] dynamically mount -H dirs into the proxy when launching

This lets you use -H unix://*.sock outside /var/run/weave. Using a
socket directly in /var/run (i.e. /var/run/foo.sock), will reintroduce
the bug with namespaces not being GCed, so that is a warning.

test/690_proxy_config_test.sh

@@ -2,7 +2,7 @@
 
 . ./config.sh
 
-start_suite "Boot the proxy should only listen on client's interface"
+start_suite "Various launch-proxy configurations"
 
 # Booting it over unix socket listens on unix socket
 run_on $HOST1 COVERAGE=$COVERAGE sudo -E weave launch-proxy
@@ -22,6 +22,11 @@ assert_raises "run_on $HOST1 sudo docker -H unix:///var/run/weave/weave.sock ps"
 assert_raises "proxy docker_on $HOST1 ps"
 weave_on $HOST1 stop-proxy
 
+# Booting it with -H outside /var/run/weave, still works
+socket="$(mktemp -d)/weave.sock"
+weave_on $HOST1 launch-proxy -H unix://$socket
+assert_raises "run_on $HOST1 sudo docker -H unix:///$socket ps" 0
+weave_on $HOST1 stop-proxy
 
 # Booting it over tls errors
 assert_raises "DOCKER_CLIENT_ARGS='--tls' weave_on $HOST1 launch-proxy" 1