fix: CMD-194 :ro → type:bind, read_only:true

Previous attempt to error if files are missing failed:
#1034 (comment)

This method succeeded, an error occurred:
> invalid mount config for type "bind": bind source path does not exist: /host_mnt/[…]/server/conf/cms/secrets.py

server/conf/docker/docker-compose-dev.all.debug.yml

@@ -6,9 +6,18 @@ services:
   cms:
     image: taccwma/core-cms:latest
     volumes:
-      - ../cms/secrets.py:/code/taccsite_cms/secrets.py:ro
-      - ../cms/settings_local.py:/code/taccsite_cms/settings_local.py:ro
-      - ../cms/settings_custom.py:/code/taccsite_cms/settings_custom.:ro
+      - type: bind
+        source: ../cms/secrets.py
+        target: /code/taccsite_cms/secrets.py
+        read_only: true
+      - type: bind
+        source: ../cms/settings_local.py
+        target: /code/taccsite_cms/settings_local.py
+        read_only: true
+      - type: bind
+        source: ../cms/settings_custom.py
+        target: /code/taccsite_cms/settings_custom.py
+        read_only: true
       - ../cms/uwsgi/uwsgi.ini:/code/uwsgi.ini
       - ../../cms/static:/code/static
       - ../../cms/media:/code/media
@@ -146,6 +155,7 @@ services:
     container_name: frontera_docs
 
 volumes:
+  core_portal_cms:
   core_portal_redis_data:
   core_portal_es_data:
   core_portal_rabbitmq_data: