headscale: promote from _dev, split README into per-task docs

Move step-by-step guides into headscale/docs/, slim README to a quick
reference, and use $HEADSCALE_CONTAINER for docker exec commands.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

headscale/compose.yaml

@@ -0,0 +1,33 @@
+services:
+  headscale:
+    image: ${HEADSCALE_IMAGE:-headscale/headscale:latest}
+    restart: ${HEADSCALE_RESTART:-unless-stopped}
+    volumes:
+      - ${VOL_PATH:-/data}/headscale/config:/etc/headscale
+      - ${VOL_PATH:-/data}/headscale/data:/var/lib/headscale
+      - ${VOL_PATH:-/data}/headscale/logs:/var/log/headscale
+    environment:
+      - TZ=${TZ:-America/Vancouver}
+      - HEADSCALE_LOG_LEVEL=${HEADSCALE_LOG_LEVEL:-info}
+    networks:
+      - proxy
+      - internal
+    command:
+      - serve
+
+  headscale-ui:
+    image: ${HEADSCALE_UI_IMAGE:-goodieshq/headscale-admin:latest}
+    restart: ${HEADSCALE_UI_RESTART:-unless-stopped}
+    environment:
+      - TZ=${TZ:-America/Vancouver}
+      - HEADSCALE_URL=http://headscale:8080
+    networks:
+      - proxy
+      - internal
+    depends_on:
+      - headscale
+
+networks:
+  proxy:
+    external: true
+  internal: