add: backend domain arg readme

README.md

@@ -9,12 +9,13 @@ docker build --tag pythonmagick - < Dockerfile.pythonmagick
 ```
 
 ### Photovoter backend + dependencies + random credentials
-```
+<pre><code>
 docker build --tag photovoter-backend \
---build-arg=USERNAME=(tr --delete --complement "[:alnum:]" </dev/random | head --bytes=5) \
+--build-arg=USERNAME=(tr --delete --complement "[:alnum:]" &lt;/dev/random | head --bytes=5) \
---build-arg=USERPASS=(tr --delete --complement "[:alnum:]" </dev/random | head --bytes=56) \
+--build-arg=USERPASS=(tr --delete --complement "[:alnum:]" &lt;/dev/random | head --bytes=56) \
+--build-arg=DOMAIN=<i>gman.me</i> \
 - < Dockerfile.photovoter-backend
-```
+</code></pre>
 
 ### Photovoter frontend
 <pre><code>