Remove cache builder stages to fix ephemeral-storage eviction

Both frontend and API pipelines had separate "Cache builder stage"
steps that built and pushed intermediate Docker targets purely for
layer caching. Running these alongside the actual build steps doubled
the Docker build work per pipeline, causing pods to be evicted when
the node hit its ephemeral-storage threshold (~20GB).

The "Build image" steps already use cache_from with the existing
:builder tags from previous builds, so the separate cache steps
are redundant for the common case (unchanged package files).

.drone.yml

@@ -21,26 +21,6 @@ steps:
       - npm ci
       - npx vitest run
 
-  - name: Cache builder stage
-    image: plugins/docker
-    resources:
-      requests:
-        memory: 2147483648
-      limits:
-        memory: 4294967296
-    settings:
-      username: viktorbarzin
-      password:
-        from_secret: dockerhub-token
-      repo: viktorbarzin/immoweb
-      dockerfile: frontend/Dockerfile
-      context: frontend
-      target: builder
-      cache_from:
-        - viktorbarzin/immoweb:builder
-      tags:
-        - builder
-
   - name: Build frontend image
     image: plugins/docker
     resources:
@@ -140,21 +120,6 @@ steps:
     commands:
       - pytest tests/ -x -q
 
-  - name: Cache builder stage
-    image: plugins/docker
-    settings:
-      username: viktorbarzin
-      password:
-        from_secret: dockerhub-token
-      repo: viktorbarzin/realestatecrawler
-      dockerfile: Dockerfile
-      context: .
-      target: builder
-      cache_from:
-        - viktorbarzin/realestatecrawler:builder
-      tags:
-        - builder
-
   - name: Build API image
     image: plugins/docker
     settings: