fix(docker): build @bete/shared before backend and discord-gateway

@bete/shared emits dist/ artifact; imports resolve via
package.json "main"/"exports" pointing to dist/. Both
services need the shared package built before `tsc` sees it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

infra/docker/Dockerfile.backend

@@ -22,6 +22,9 @@ COPY services/backend ./services/backend
 # Install dependencies
 RUN pnpm install --frozen-lockfile
 
+# Build shared workspace dependency first
+RUN pnpm --filter './packages/shared' run build
+
 # Build backend
 RUN pnpm --filter './services/backend' run build
 

infra/docker/Dockerfile.discord-gateway

@@ -28,6 +28,9 @@ COPY services/discord-gateway/drizzle ./drizzle
 # Install dependencies
 RUN pnpm install --frozen-lockfile
 
+# Build shared workspace dependency first
+RUN pnpm --filter './packages/shared' run build
+
 # Build discord gateway
 RUN pnpm --filter './services/discord-gateway' run build