Fix docker-compose source volume definition

Always use a bind mount for the source volume to ensure the source-push volume works as intended.

Additionally add some clarifying comments in several places.

doc/docker/docker-compose.yml

@@ -1,8 +1,18 @@
 version: '3'
 volumes:
+  # NOTE: all directories referenced by "device" entries below need to be created manually before starting the containers
+  # egroupware sources
   sources:
-  # you can use either an anonymous volume, or explicitly specify an existing (!) path
-  # device: $PWD/sources
+    driver_opts:
+      type: none
+      o: bind
+      device: $PWD/sources/
+  # sources for push server, swoolepush is a subdirectory of egroupware sources (within volume "sources")
+  sources-push:
+    driver_opts:
+      type: none
+      o: bind
+      device: $PWD/sources/egroupware/swoolepush
   db:
   data:
     driver_opts:
@@ -21,16 +31,11 @@ volumes:
   #    # location of deprecated EGroupware packages like Wiki, SiteMgr, KnowledgeBase
   #    device: /usr/share/egroupware
   #    #device: $PWD/extra
-  # sources for push server, swoolepush subdirectory of egroupware sources, in volume "sources"
-  sources-push:
-    driver_opts:
-      type: none
-      o: bind
-      device: $PWD/sources/egroupware/swoolepush
   # volume to store config.inc.php file / token shared between egroupware and push container
   push-config:
   sessions:
   # collabora-config directory, initially filled by collabora-init container
+  # additionally some more configuration files are needed in order for collabora to work, which are _not_ generated by the collabora-init container
   collabora-config:
     driver_opts:
       type: none
@@ -260,4 +265,4 @@ services:
 #    volumes:
 #      - /var/run/docker.sock:/var/run/docker.sock
 #      - portainer_data:/data
-#    container_name: portainer
+#    container_name: portainer