CI: add actions/set-docker-config-dir to set DOCKER_CONFIG (#8676)

## Problem

In several workflows, we have repeating code which is separated into 
two steps:
```bash
mkdir -p $(pwd)/.docker-custom
echo DOCKER_CONFIG=/tmp/.docker-custom >> $GITHUB_ENV
...
rm -rf $(pwd)/.docker-custom
```

Such copy-paste is prone to errors; for example, in one case, instead of
`$(pwd)/.docker-custom`, we use `/tmp/.docker-custom`, which is shared
between workflows.

## Summary of changes
- Create a new action `actions/set-docker-config-dir`, which sets
`DOCKER_CONFIG` and deletes it in a Post action part

.github/workflows/build-build-tools-image.yml

@@ -56,13 +56,7 @@ jobs:
 
       - uses: actions/checkout@v4
 
-      # Use custom DOCKER_CONFIG directory to avoid conflicts with default settings
-      # The default value is ~/.docker
-      - name: Set custom docker config directory
-        run: |
-          mkdir -p /tmp/.docker-custom
-          echo DOCKER_CONFIG=/tmp/.docker-custom >> $GITHUB_ENV
-
+      - uses: ./.github/actions/set-docker-config-dir
       - uses: docker/setup-buildx-action@v3
         with:
           cache-binary: false
@@ -89,11 +83,6 @@ jobs:
           cache-to: ${{ github.ref_name == 'main' && format('type=registry,ref=cache.neon.build/build-tools:cache-{0},mode=max', matrix.arch) || '' }}
           tags: neondatabase/build-tools:${{ inputs.image-tag }}-${{ matrix.arch }}
 
-      - name: Remove custom docker config directory
-        if: always()
-        run: |
-          rm -rf /tmp/.docker-custom
-
   merge-images:
     needs: [ build-image ]
     runs-on: ubuntu-22.04

.github/workflows/build_and_test.yml

@@ -484,12 +484,7 @@ jobs:
           submodules: true
           fetch-depth: 0
 
-      # Use custom DOCKER_CONFIG directory to avoid conflicts with default settings
-      # The default value is ~/.docker
-      - name: Set custom docker config directory
-        run: |
-          mkdir -p .docker-custom
-          echo DOCKER_CONFIG=$(pwd)/.docker-custom >> $GITHUB_ENV
+      - uses: ./.github/actions/set-docker-config-dir
       - uses: docker/setup-buildx-action@v3
         with:
           cache-binary: false
@@ -521,11 +516,6 @@ jobs:
           tags: |
             neondatabase/neon:${{ needs.tag.outputs.build-tag }}-${{ matrix.arch }}
 
-      - name: Remove custom docker config directory
-        if: always()
-        run: |
-          rm -rf .docker-custom
-
   neon-image:
     needs: [ neon-image-arch, tag ]
     runs-on: ubuntu-22.04
@@ -570,12 +560,7 @@ jobs:
           submodules: true
           fetch-depth: 0
 
-      # Use custom DOCKER_CONFIG directory to avoid conflicts with default settings
-      # The default value is ~/.docker
-      - name: Set custom docker config directory
-        run: |
-          mkdir -p .docker-custom
-          echo DOCKER_CONFIG=$(pwd)/.docker-custom >> $GITHUB_ENV
+      - uses: ./.github/actions/set-docker-config-dir
       - uses: docker/setup-buildx-action@v3
         with:
           cache-binary: false
@@ -658,11 +643,6 @@ jobs:
           tags: |
             neondatabase/compute-tools:${{ needs.tag.outputs.build-tag }}-${{ matrix.arch }}
 
-      - name: Remove custom docker config directory
-        if: always()
-        run: |
-          rm -rf .docker-custom
-
   compute-node-image:
     needs: [ compute-node-image-arch, tag ]
     runs-on: ubuntu-22.04
@@ -735,13 +715,7 @@ jobs:
           curl -fL https://github.com/neondatabase/autoscaling/releases/download/$VM_BUILDER_VERSION/vm-builder -o vm-builder
           chmod +x vm-builder
 
-      # Use custom DOCKER_CONFIG directory to avoid conflicts with default settings
-      # The default value is ~/.docker
-      - name: Set custom docker config directory
-        run: |
-          mkdir -p .docker-custom
-          echo DOCKER_CONFIG=$(pwd)/.docker-custom >> $GITHUB_ENV
-
+      - uses: ./.github/actions/set-docker-config-dir
       - uses: docker/login-action@v3
         with:
           username: ${{ secrets.NEON_DOCKERHUB_USERNAME }}
@@ -764,11 +738,6 @@ jobs:
         run: |
           docker push neondatabase/vm-compute-node-${{ matrix.version }}:${{ needs.tag.outputs.build-tag }}
 
-      - name: Remove custom docker config directory
-        if: always()
-        run: |
-          rm -rf .docker-custom
-
   test-images:
     needs: [ check-permissions, tag, neon-image, compute-node-image ]
     strategy:
@@ -784,13 +753,7 @@ jobs:
         with:
           fetch-depth: 0
 
-      # Use custom DOCKER_CONFIG directory to avoid conflicts with default settings
-      # The default value is ~/.docker
-      - name: Set custom docker config directory
-        run: |
-          mkdir -p .docker-custom
-          echo DOCKER_CONFIG=$(pwd)/.docker-custom >> $GITHUB_ENV
-
+      - uses: ./.github/actions/set-docker-config-dir
       - uses: docker/login-action@v3
         with:
           username: ${{ secrets.NEON_DOCKERHUB_USERNAME }}
@@ -830,11 +793,6 @@ jobs:
           docker compose -f ./docker-compose/docker-compose.yml logs || 0
           docker compose -f ./docker-compose/docker-compose.yml down
 
-      - name: Remove custom docker config directory
-        if: always()
-        run: |
-          rm -rf .docker-custom
-
   promote-images:
     permissions:
       contents: read  # This is required for actions/checkout