Improve the docker experience

- Only one env variable to configure on first usage
- Auto-generate a default config
- Improve doc

docs/getting-started.md

@@ -37,6 +37,8 @@ Install via:
 See the [Latest release](https://github.com/kamax-io/mxisd/releases/latest) for links to each.
 
 ## Configure
+**NOTE**: please view the install instruction for your platform, as this step might be optional/handled for you.
+
 Create/edit a minimal configuration (see installer doc for the location):
 ```
 matrix.domain: 'MyMatrixDomain.org'

docs/install/docker.md

@@ -5,10 +5,18 @@ Pull the latest stable image:
 docker pull kamax/mxisd
 ```
 
+## Configure
+On first run, simply using `MATRIX_DOMAIN` as an environment variable will create a default config for you.  
+You can also provide a configuration file named `mxisd.yaml` in the volume mapped to `/etc/mxisd` before starting your
+container.
+
 ## Run
-Run it (adapt volume paths to your host):
+Use the following command after adapting to your needs:
+- The `MATRIX_DOMAIN` environment variable to yours
+- The volumes host paths
+
 ```
-docker run --rm -v /data/mxisd/etc:/etc/mxisd -v /data/mxisd/var:/var/mxisd -p 8090:8090 -t kamax/mxisd
+docker run --rm -e MATRIX_DOMAIN=example.org -v /data/mxisd/etc:/etc/mxisd -v /data/mxisd/var:/var/mxisd -p 8090:8090 -t kamax/mxisd
 ```
 
 For more info, including the list of possible tags, see [the public repository](https://hub.docker.com/r/kamax/mxisd/)