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/firefly-iii/docker-compose.yaml

@@ -6,7 +6,7 @@ services:
     depends_on:
       - mariadb
     env_file:
-      - .env.secrets   # APP_KEY, DB_PASSWORD — never committed
+      - .env.secrets
     environment:
       - DB_CONNECTION=mysql
       - DB_HOST=mariadb
@@ -30,7 +30,7 @@ services:
     container_name: fireflyiii_db
     restart: unless-stopped
     env_file:
-      - .env.secrets   # MYSQL_PASSWORD, MYSQL_ROOT_PASSWORD — never committed
+      - .env.secrets
     environment:
       - PUID=${PUID}
       - PGID=${PGID}