Add a DOCKER_API_VERSION env var
Closes:
Just for  :-)
Fixed some comment formatting too while in there.
@@ -38,6 +38,7 @@ the [installation](../../installation/index.md) instructions for your operating
For easy reference, the following list of environment variables are supported
by the `docker` command line:
+* `DOCKER_API_VERSION` The API version to use (e.g. `1.19`)
* `DOCKER_CONFIG` The location of your client configuration files.
* `DOCKER_CERT_PATH` The location of your authentication keys.
* `DOCKER_DRIVER` The graph driver to use.
