fix(docker): use pnpm --filter to build each service in Dockerfiles

Root scripts point to old paths (src/, frontend/) that don't exist in the
services/ monorepo layout. Use pnpm --filter to invoke each service's
own build script (tsc) instead.

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

infra/docker/Dockerfile.backend

@@ -20,7 +20,7 @@ COPY services/backend ./services/backend
 RUN pnpm install --frozen-lockfile
 
 # Build backend
-RUN pnpm run build:server
+RUN pnpm --filter './services/backend' run build
 
 # Expose port
 EXPOSE 3001

infra/docker/Dockerfile.discord-gateway

@@ -20,7 +20,7 @@ COPY services/discord-gateway ./services/discord-gateway
 RUN pnpm install --frozen-lockfile
 
 # Build discord gateway
-RUN pnpm run build
+RUN pnpm --filter './services/discord-gateway' run build
 
 # Create recordings directory
 RUN mkdir -p /app/recordings

infra/docker/Dockerfile.frontend

@@ -20,7 +20,7 @@ COPY services/frontend ./services/frontend
 RUN pnpm install --frozen-lockfile
 
 # Build frontend
-RUN pnpm run build:web
+RUN pnpm --filter './services/frontend' run build
 
 # Expose port (served by backend)
 EXPOSE 3000