perf: speed up docker builds (#12859)

- Use separate keys to avoid cache overwrites across different
architecture builds


https://linear.app/chatwoot/issue/CW-5945/perf-speed-up-docker-builds

### 25 mins  ---> 5mins


## before

<img width="971" height="452" alt="image"
src="https://github.com/user-attachments/assets/535cebd6-6c16-48d1-a62d-ffb6f2fc9b08"
/>


## after
<img width="940" height="428" alt="image"
src="https://github.com/user-attachments/assets/359eb313-4bb5-4e0e-9492-a8ad48645159"
/>

.github/workflows/test_docker_build.yml

@@ -36,5 +36,5 @@ jobs:
           platforms: ${{ matrix.platform }}
           push: false
           load: false
-          cache-from: type=gha
-          cache-to: type=gha,mode=max
+          cache-from: type=gha,scope=${{ matrix.platform }}
+          cache-to: type=gha,mode=max,scope=${{ matrix.platform }}