chore: speed up Docker builds

- drop arm64/QEMU — build amd64 only
- add pnpm store cache mounts to client and server install steps

.github/workflows/container-image.yml

@@ -97,9 +97,6 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f # v3
 
-      - name: Set up QEMU
-        uses: docker/setup-qemu-action@c7c53464625b32c7a7e944ae62b3e17d2b600130 # v3
-
       - name: Log in to GHCR
         uses: docker/login-action@c94ce9fb468520275223c153574b00df6fe4bcc9 # v3
         with:
@@ -115,6 +112,6 @@ jobs:
           file: ./Dockerfile
           push: true
           tags: ${{ steps.vars.outputs.tags }}
-          platforms: linux/amd64,linux/arm64
+          platforms: linux/amd64
           cache-from: type=gha
           cache-to: type=gha,mode=max