build(compose): Type safe and consistent YAML

- Avoid YAML type conversion/casting magic:

  https://www.rpatterson.net/blog/docker-gotchas/#yaml-needs-some-zen
  https://sidneyliebrand.medium.com/the-greatnesses-and-gotchas-of-yaml-5e3377ef0c55

- Prefer mappings over `"...=..."` strings such as for `environment:`:

  YAML mappings better describe the declaritive intention of those parts of the
  configuration. Use mappings where order doesn't matter and YAML arrays/lists make
  confuguration less readable.

docker/testing/docker-compose.yml

@@ -1,39 +1,39 @@
 services:
   ytdl-sub-gui:
-    image: ytdl-sub-gui:local
-    container_name: ytdl-sub-gui
+    image: "ytdl-sub-gui:local"
+    container_name: "ytdl-sub-gui"
     environment:
-      - PUID=1000
-      - PGID=1000
-      - TZ=America/Los_Angeles
-      - CRON_SCHEDULE="*/1 * * * *"
-      - CRON_RUN_ON_START=true
+      PUID: "1000"
+      PGID: "1000"
+      TZ: "America/Los_Angeles"
+      CRON_SCHEDULE: '"*/1 * * * *"'
+      CRON_RUN_ON_START: "true"
     volumes:
-      - ./volumes/ytdl-sub-gui:/config
+      - "./volumes/ytdl-sub-gui/:/config/"
     ports:
-      - 8443:8443
-    restart: unless-stopped
+      - "8443:8443"
+    restart: "unless-stopped"
   ytdl-sub:
-    image: ytdl-sub:local
-    container_name: ytdl-sub
+    image: "ytdl-sub:local"
+    container_name: "ytdl-sub"
     environment:
-      - PUID=1000
-      - PGID=1000
-      - TZ=America/Los_Angeles
-      - CRON_SCHEDULE="*/1 * * * *"
-      - CRON_RUN_ON_START=true
+      PUID: "1000"
+      PGID: "1000"
+      TZ: "America/Los_Angeles"
+      CRON_SCHEDULE: '"*/1 * * * *"'
+      CRON_RUN_ON_START: "true"
     volumes:
-      - ./volumes/ytdl-sub:/config
-    restart: unless-stopped
+      - "./volumes/ytdl-sub/:/config/"
+    restart: "unless-stopped"
   ytdl-sub-ubuntu:
-    image: ytdl-sub-ubuntu:local
-    container_name: ytdl-sub-ubuntu
+    image: "ytdl-sub-ubuntu:local"
+    container_name: "ytdl-sub-ubuntu"
     environment:
-      - PUID=1000
-      - PGID=1000
-      - TZ=America/Los_Angeles
-      - CRON_SCHEDULE="*/1 * * * *"
-      - CRON_RUN_ON_START=true
+      PUID: "1000"
+      PGID: "1000"
+      TZ: "America/Los_Angeles"
+      CRON_SCHEDULE: '"*/1 * * * *"'
+      CRON_RUN_ON_START: "true"
     volumes:
-      - ./volumes/ytdl-sub:/config
-    restart: unless-stopped
+      - "./volumes/ytdl-sub/:/config/"
+    restart: "unless-stopped"