Added docker exec completion, introduced in docker 1.3

plugins/docker/_docker

@@ -61,6 +61,14 @@ __events() {
         '--since=[Show previously created events and then stream.]'
 }
 
+__exec() {
+  _arguments \
+    '-d[Detached mode: run command in the background]' \
+    '-i[Keep STDIN open even if not attached]' \
+    '-t[Allocate a pseudo-TTY]'
+  __docker_containers
+}
+
 __export() {
    __docker_containers
 }
@@ -249,6 +257,7 @@ _1st_arguments=(
     "cp":"Copy files/folders from the containers filesystem to the host path"
     "diff":"Inspect changes on a container's filesystem"
     "events":"Get real time events from the server"
+    "exec":"Run a command in an existing container"
     "export":"Stream the contents of a container as a tar archive"
     "history":"Show the history of an image"
     "images":"List images"
@@ -299,6 +308,8 @@ case "$words[1]" in
         __diff ;;
     events)
         __events ;;
+    exec)
+        __exec ;;
     export)
         __export ;;
     history)