docs: build contexts and bake options

Signed-off-by: Andrei Baibaratsky <andrei@baibaratsky.com>

docs/advanced/export-docker.md

@@ -33,3 +33,75 @@ jobs:
         run: |
           docker image inspect myimage:latest
 ```
+
+
+## Usage of the built image in other build steps
+
+[By default](https://github.com/docker/setup-buildx-action#customizing), `docker/setup-buildx-action@v2` 
+uses `docker-container` as a build driver, so the docker images are not available in the builder container.
+To use them, you may use [build contexts](https://github.com/docker/buildx/blob/master/docs/reference/buildx_build.md#-additional-build-contexts---build-context):
+
+```yaml
+name: ci
+
+on:
+  push:
+    branches:
+      - 'main'
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      -
+        name: Checkout
+        uses: actions/checkout@v3
+      -
+        name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v2
+      -
+        name: Build base image
+        uses: docker/build-push-action@v3
+        with:
+          context: base
+          load: true
+          tags: my-base-image:latest
+      -
+        name: Build image from my-base-image:latest
+        uses: docker/build-push-action@v3
+        with:
+          context: .
+          build-contexts: |
+            base-image=docker-image://my-base-image:latest
+          tags: myimage:latest
+```
+
+Where `base-image`is the name of the base image (or stage name if specified) in your Dockerfile:
+```Dockerfile
+FROM base-image
+```
+
+### Bake alternative
+
+You may also want to use [bake](https://docs.docker.com/build/bake/build-contexts/#using-a-result-of-one-target-as-a-base-image-in-another-target)
+and build the base image and the target image in one build step:
+```terraform
+# docker-bake.hcl
+target "base" {
+  dockerfile = "baseapp.Dockerfile"
+}
+
+target "app" {
+  contexts = {
+    baseapp = "target:base"
+  }
+}
+```
+
+```yaml
+      -
+        name: Build
+        uses: docker/bake-action@v2
+        with:
+          target: app
+```