Enhance documentation

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

docs/advanced/isolated-builders.md

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+# Isolated builders
+
+```yaml
+name: ci
+
+on:
+  push:
+    branches:
+      - 'master'
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      -
+        name: Checkout
+        uses: actions/checkout@v2
+      -
+        uses: docker/setup-buildx-action@v1
+        id: builder1
+      -
+        uses: docker/setup-buildx-action@v1
+        id: builder2
+      -
+        name: Builder 1 name
+        run: echo ${{ steps.builder1.outputs.name }}
+      -
+        name: Builder 2 name
+        run: echo ${{ steps.builder2.outputs.name }}
+      -
+        name: Build against builder1
+        uses: docker/build-push-action@v2
+        with:
+          builder: ${{ steps.builder1.outputs.name }}
+          context: .
+          target: mytarget1
+      -
+        name: Build against builder2
+        uses: docker/build-push-action@v2
+        with:
+          builder: ${{ steps.builder2.outputs.name }}
+          context: .
+          target: mytarget2
+```