Explain docker install, allow CONFIG from env

README.md

@@ -23,5 +23,10 @@ A sample configration that configures the smtp server is provided in `scansmb.ex
 
 Start it on the command line like this (this example makes use of all means of configuration):
 ```
-PRINTER_HOST=192.168.8.111 python3 scansmb.py -c scansmb.conf -f [email protected] -t [email protected]
+PRINTER_HOST=192.168.8.111 python3 scansmb.py -c scansmb.conf -f [email protected] -t [email protected]
+```
+
+Or use the docker image at [dockerhub](https://hub.docker.com/r/hensur/scansmb):
+```
+docker run -v $(pwd)/scansmb.conf:/scansmb.conf -e PRINTER_HOST=192.168.8.111 -e [email protected] -e [email protected] -e CONFIG=/scansmb.conf hensur/scansmb
 ```

scansmb.py

@@ -105,7 +105,7 @@ def loop(ctx, options):
 
 def main():
     parser = configargparse.ArgParser(default_config_files=['scansmb.conf'])
-    parser.add("-c", "--config", is_config_file=True, help="config file path")
+    parser.add("-c", "--config", is_config_file=True, help="config file path", env_var="CONFIG")
     parser.add("-p", "--printer-host", required=True,
                help="printer hostname", env_var="PRINTER_HOST")
     parser.add("--smtp-user", required=True,