Merge pull request #721 from crazy-max/compose-ext

bake: `x-bake` extension field with compose

docs/reference/buildx_bake.md

@@ -756,3 +756,108 @@ $ docker buildx bake --print app
   }
 }
 ```
+
+### Extension field with Compose
+
+[Special extension](https://github.com/compose-spec/compose-spec/blob/master/spec.md#extension)
+field `x-bake` can be used in your compose file to evaluate fields that are not
+(yet) available in the [build definition](https://github.com/compose-spec/compose-spec/blob/master/build.md#build-definition).
+
+```yaml
+# docker-compose.yml
+services:
+  addon:
+    image: ct-addon:bar
+    build:
+      context: .
+      dockerfile: ./Dockerfile
+      args:
+        CT_ECR: foo
+        CT_TAG: bar
+      x-bake:
+        tags:
+          - ct-addon:foo
+          - ct-addon:alp
+        platforms:
+          - linux/amd64
+          - linux/arm64
+        cache-from:
+          - user/app:cache
+          - type=local,src=path/to/cache
+        cache-to: type=local,dest=path/to/cache
+        pull: true
+
+  aws:
+    image: ct-fake-aws:bar
+    build:
+      dockerfile: ./aws.Dockerfile
+      args:
+        CT_ECR: foo
+        CT_TAG: bar
+      x-bake:
+        secret:
+          - id=mysecret,src=./secret
+          - id=mysecret2,src=./secret2
+        platforms: linux/arm64
+        output: type=docker
+        no-cache: true
+```
+
+```console
+$ docker buildx bake --print
+{
+  "target": {
+    "addon": {
+      "context": ".",
+      "dockerfile": "./Dockerfile",
+      "args": {
+        "CT_ECR": "foo",
+        "CT_TAG": "bar"
+      },
+      "tags": [
+        "ct-addon:foo",
+        "ct-addon:alp"
+      ],
+      "cache-from": [
+        "user/app:cache",
+        "type=local,src=path/to/cache"
+      ],
+      "cache-to": [
+        "type=local,dest=path/to/cache"
+      ],
+      "platforms": [
+        "linux/amd64",
+        "linux/arm64"
+      ],
+      "pull": true
+    },
+    "aws": {
+      "context": ".",
+      "dockerfile": "./aws.Dockerfile",
+      "args": {
+        "CT_ECR": "foo",
+        "CT_TAG": "bar"
+      },
+      "tags": [
+        "ct-fake-aws:bar"
+      ],
+      "secret": [
+        "id=mysecret,src=./secret",
+        "id=mysecret2,src=./secret2"
+      ],
+      "platforms": [
+        "linux/arm64"
+      ],
+      "output": [
+        "type=docker"
+      ],
+      "no-cache": true
+    }
+  }
+}
+```
+
+Complete list of valid fields for `x-bake`:
+
+`tags`, `cache-from`, `cache-to`, `secret`, `ssh`, `platforms`, `output`,
+`pull`, `no-cache`