Add ability to add multiple tags with docker build

Signed-off-by: Shijiang Wei <mountkin@gmail.com>

docs/reference/commandline/build.md

@@ -31,7 +31,7 @@ parent = "smn_cli"
       --pull=false                    Always attempt to pull a newer version of the image
       -q, --quiet=false               Suppress the verbose output generated by the containers
       --rm=true                       Remove intermediate containers after a successful build
-      -t, --tag=""                    Repository name (and optionally a tag) for the image
+      -t, --tag=[]                    Name and optionally a tag in the 'name:tag' format
       --ulimit=[]                     Ulimit options
 
 Builds Docker images from a Dockerfile and a "context". A build's context is
@@ -227,6 +227,14 @@ uploaded context. The builder reference contains detailed information on
 This will build like the previous example, but it will then tag the resulting
 image. The repository name will be `vieux/apache` and the tag will be `2.0`
 
+You can apply multiple tags to an image. For example, you can apply the `latest`
+tag to a newly built image and add another tag that references a specific
+version.
+For example, to tag an image both as `whenry/fedora-jboss:latest` and
+`whenry/fedora-jboss:v2.1`, use the following:
+
+    $ docker build -t whenry/fedora-jboss:latest -t whenry/fedora-jboss:v2.1 .
+
 ### Specify Dockerfile (-f)
 
     $ docker build -f Dockerfile.debug .