Remove unused workflow to build docker images

Implied by f48166ff36 (update-dependencies: remove things used for Cirrus,
2026-05-29).

See #12626

.github/workflows/build_docker_images.yml

@@ -1,64 +0,0 @@
-name: Build Docker test images
-
-on:
-  push:
-    branches:
-      - master
-    paths:
-      - 'docker/**'
-  workflow_dispatch:
-
-concurrency:
-  group: docker-builds
-
-env:
-  REGISTRY: ghcr.io
-  NAMESPACE: fish-ci
-
-jobs:
-  docker-build:
-    if: github.repository_owner == 'fish-shell'
-    permissions:
-      contents: read
-      packages: write
-      attestations: write
-      id-token: write
-
-    strategy:
-      fail-fast: false
-      matrix:
-        include:
-          - os: ubuntu-latest
-            target: alpine
-          - os: ubuntu-latest
-            target: ubuntu-oldest-supported
-    runs-on: ${{ matrix.os }}
-
-    steps:
-      -
-        name: Checkout
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2, build_tools/update-dependencies.sh
-      -
-        name: Login to Container registry
-        uses: docker/login-action@650006c6eb7dba73a995cc03b0b2d7f5ca915bee # v4.2.0, build_tools/update-dependencies.sh
-        with:
-          registry: ${{ env.REGISTRY }}
-          username: ${{ github.actor }}
-          password: ${{ secrets.GITHUB_TOKEN }}
-      -
-        name: Extract metadata (tags, labels) for Docker
-        id: meta
-        uses: docker/metadata-action@80c7e94dd9b9319bd5eb7a0e0fe9291e23a2a2e9 # v6.1.0, build_tools/update-dependencies.sh
-        with:
-          images: ${{ env.REGISTRY }}/${{ github.repository_owner }}/${{ env.NAMESPACE }}/${{ matrix.target }}
-          flavor: |
-            latest=true
-      -
-        name: Build and push
-        uses: docker/build-push-action@f9f3042f7e2789586610d6e8b85c8f03e5195baf # v7.2.0, build_tools/update-dependencies.sh
-        with:
-          context: docker/context
-          push: true
-          file: docker/${{ matrix.target }}.Dockerfile
-          tags: ${{ steps.meta.outputs.tags }}
-          labels: ${{ steps.meta.outputs.labels }}