fix: switch Docker images from Alpine to Debian slim for native addon compatibility

Alpine (musl libc) is incompatible with matrix-sdk-crypto-nodejs native binary
which requires glibc's ld-linux-x86-64.so.2. Switched all Node.js Dockerfiles
to node:24-slim (Debian/glibc). Also fixed docker-compose.matrix.yml network
naming from undefined mosaic-network to mosaic-internal.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker/docker-compose.matrix.yml

@@ -60,7 +60,7 @@ services:
       postgres:
         condition: service_healthy
     networks:
-      - mosaic-network
+      - mosaic-internal
 
   # ======================
   # Synapse (Matrix Homeserver)
@@ -76,9 +76,9 @@ services:
       - "${SYNAPSE_CLIENT_PORT:-8008}:8008"
       - "${SYNAPSE_FEDERATION_PORT:-8448}:8448"
     volumes:
-      - ./matrix/synapse/homeserver.yaml:/data/homeserver.yaml:ro
-      - synapse_data:/data/media_store
-      - synapse_signing_key:/data/keys
+      - /opt/mosaic/synapse/homeserver.yaml:/data/homeserver.yaml:ro
+      - /opt/mosaic/synapse/media_store:/data/media_store
+      - /opt/mosaic/synapse/keys:/data/keys
     depends_on:
       postgres:
         condition: service_healthy
@@ -91,7 +91,7 @@ services:
       retries: 5
       start_period: 30s
     networks:
-      - mosaic-network
+      - mosaic-internal
     labels:
       com.mosaic.service: "matrix-synapse"
       com.mosaic.description: "Matrix homeserver (dev)"
@@ -106,7 +106,7 @@ services:
     ports:
       - "${ELEMENT_PORT:-8501}:80"
     volumes:
-      - ./matrix/element/config.json:/app/config.json:ro
+      - /opt/mosaic/synapse/element-config.json:/app/config.json:ro
     depends_on:
       synapse:
         condition: service_healthy
@@ -117,13 +117,7 @@ services:
       retries: 3
       start_period: 10s
     networks:
-      - mosaic-network
+      - mosaic-internal
     labels:
       com.mosaic.service: "matrix-element"
       com.mosaic.description: "Element Web client (dev)"
-
-volumes:
-  synapse_data:
-    name: mosaic-synapse-data
-  synapse_signing_key:
-    name: mosaic-synapse-signing-key