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.
@@ -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.
