ci: [torrust#634] build docker image only once

Removed the previous docker build becuase the cache is not being used in
the runner build command. It builds again the image.

todo: docker layers cache

Author: josecelano

.github/workflows/testing.yaml

@@ -121,27 +121,7 @@ jobs:
         toolchain: [nightly]
 
     steps:
-      - id: setup-docker
-        name: Setup Toolchain
-        uses: docker/setup-buildx-action@v3
-
-      - id: build
-        name: Build
-        uses: docker/build-push-action@v5
-        with:
-          file: ./Containerfile
-          push: false
-          load: true
-          target: release
-          tags: torrust-tracker:local
-          cache-from: type=gha
-          cache-to: type=gha
-
-      - id: inspect
-        name: Inspect
-        run: docker image inspect torrust-tracker:local
-
-      - id: setup-rust
+      - id: setup
         name: Setup Toolchain
         uses: dtolnay/rust-toolchain@stable
         with: