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>

apps/web/Dockerfile

@@ -2,7 +2,9 @@
 # Enable BuildKit features for cache mounts
 
 # Base image for all stages
-FROM node:24-alpine AS base
+# Uses Debian slim (glibc) for consistency with API/orchestrator and to prevent
+# future native addon compatibility issues with Alpine's musl libc.
+FROM node:24-slim AS base
 
 # Install pnpm globally
 RUN corepack enable && corepack prepare pnpm@10.27.0 --activate
@@ -75,7 +77,7 @@ RUN mkdir -p ./apps/web/public
 # ======================
 # Production stage
 # ======================
-FROM node:24-alpine AS production
+FROM node:24-slim AS production
 
 # Remove npm (unused in production — we use pnpm) to reduce attack surface
 RUN rm -rf /usr/local/lib/node_modules/npm /usr/local/bin/npm /usr/local/bin/npx
@@ -84,10 +86,11 @@ RUN rm -rf /usr/local/lib/node_modules/npm /usr/local/bin/npm /usr/local/bin/npx
 RUN corepack enable && corepack prepare pnpm@10.27.0 --activate
 
 # Install dumb-init for proper signal handling
-RUN apk add --no-cache dumb-init
+RUN apt-get update && apt-get install -y --no-install-recommends dumb-init \
+    && rm -rf /var/lib/apt/lists/*
 
 # Create non-root user
-RUN addgroup -g 1001 -S nodejs && adduser -S nextjs -u 1001
+RUN groupadd -g 1001 nodejs && useradd -m -u 1001 -g nodejs nextjs
 
 WORKDIR /app