Add some info about what environment variables are available

Having the list in one spot makes it easier for people to see what's
avaiable instead of having to scan all of the docs and extract the info.

Signed-off-by: Doug Davis <dug@us.ibm.com>

docs/sources/reference/commandline/cli.md

@@ -17,6 +17,30 @@ or execute `docker help`:
 
       ...
 
+## Environment Variables
+
+For easy reference, the following list of environment variables are supported
+by the `docker` command line:
+
+* `DOCKER_CERT_PATH` The location of your authentication keys.
+* `DOCKER_DRIVER` The graph driver to use.
+* `DOCKER_HOST` Daemon socket to connect to.
+* `DOCKER_NOWARN_KERNEL_VERSION` Prevent warnings that your Linux kernel is unsuitable for Docker.
+* `DOCKER_RAMDISK` If set this will disable 'pivot_root'.
+* `DOCKER_TLS_VERIFY` When set Docker uses TLS and verifies the remote.
+* `DOCKER_TMPDIR` Location for temporary Docker files.
+
+Because Docker is developed using 'Go', you can also use any environment
+variables used by the 'Go' runtime. In particular, you may find these useful:
+
+* `HTTP_PROXY`
+* `HTTPS_PROXY`
+* `NO_PROXY`
+
+These Go environment variables are case-insensitive. See the
+[Go specification](http://golang.org/pkg/net/http/) for details on these
+variables.
+
 ## Help
 To list the help on any command just execute the command, followed by the `--help` option.
 
@@ -539,7 +563,7 @@ Instead of specifying a context, you can pass a single Dockerfile in the
 	docker build - < Dockerfile
 
 If you use STDIN or specify a `URL`, the system places the contents into a
-file called `Dockerfile`, and any `-f`, `--file` option is ignored. In this 
+file called `Dockerfile`, and any `-f`, `--file` option is ignored. In this
 scenario, there is no context.
 
 ### Return code
@@ -795,7 +819,7 @@ relative to the root of the container's filesystem.
 
     Usage: docker cp CONTAINER:PATH HOSTDIR|-
 
-    Copy files/folders from the PATH to the HOSTDIR. 
+    Copy files/folders from the PATH to the HOSTDIR.
 
 
 ## create
@@ -1530,7 +1554,7 @@ just a specific mapping:
       --before=""           Show only container created before Id or Name
       -f, --filter=[]       Filter output based on conditions provided
       -l, --latest=false    Show the latest created container, include non-running
-      -n=-1                 Show n last created containers, include non-running 
+      -n=-1                 Show n last created containers, include non-running
       --no-trunc=false      Don't truncate output
       -q, --quiet=false     Only display numeric IDs
       -s, --size=false      Display total file sizes
@@ -1946,7 +1970,7 @@ format:
     com.example.label2=another\ label
     com.example.label3
 
-You can load multiple label-files by supplying multiple  `--label-file` flags. 
+You can load multiple label-files by supplying multiple  `--label-file` flags.
 
 For additional information on working with labels, see [*Labels - custom
 metadata in Docker*](/userguide/labels-custom-metadata/) in the Docker User
@@ -2060,7 +2084,7 @@ application change:
 
 #### Restart Policies
 
-Use Docker's `--restart` to specify a container's *restart policy*. A restart 
+Use Docker's `--restart` to specify a container's *restart policy*. A restart
 policy controls whether the Docker daemon restarts a container after exit.
 Docker supports the following restart policies:
 
@@ -2075,7 +2099,7 @@ Docker supports the following restart policies:
     <tr>
       <td><strong>no</strong></td>
       <td>
-        Do not automatically restart the container when it exits. This is the 
+        Do not automatically restart the container when it exits. This is the
         default.
       </td>
     </tr>
@@ -2087,7 +2111,7 @@ Docker supports the following restart policies:
       </td>
       <td>
         Restart only if the container exits with a non-zero exit status.
-        Optionally, limit the number of restart retries the Docker 
+        Optionally, limit the number of restart retries the Docker
         daemon attempts.
       </td>
     </tr>
@@ -2107,7 +2131,7 @@ Docker supports the following restart policies:
 This will run the `redis` container with a restart policy of **always**
 so that if the container exits, Docker will restart it.
 
-More detailed information on restart policies can be found in the 
+More detailed information on restart policies can be found in the
 [Restart Policies (--restart)](/reference/run/#restart-policies-restart) section
 of the Docker run reference page.