stacks: add nextcloud Dockerfiles, fix a broken kodi compose edit

Adds the two Dockerfiles nextcloud's compose build: contexts were
already referencing but never had (custom smbclient support for the
main container, a documentserver base for euro-office) - both had
their real internal-CA-trust mechanism replaced with a comment
explaining what's there and why, per the standing rule against
publishing that detail.

Also fixes a compose bug: kodi-ma-bridge's `environment:` key had
been dropped, leaving KEY=value pairs nested under `env_file:` (which
expects file paths, not inline vars) - would have made docker-compose
try to open a file literally named `TZ=${TZ}`.

Rest of this commit is manual cleanup (comment tidying, dropped
personal media-library volume mounts, a couple of .env comment
trims).

stacks/productivity/joplin/.env

@@ -6,7 +6,7 @@ POSTGRES_PORT=5432
 TZ=Etc/UTC
 
 # User Credentials
-APP_BASE_URL=https://notes.example.com
+APP_BASE_URL=https://joplin.example.com
 
 # Admin account seed (only read on a fresh first-ever boot, e.g. after a full recreate)
 APP_ADMIN_EMAIL=[email protected]