Add subdirectory for Git context

Since v0.9.0 of BuildKit (BuildX v0.7.0) you can provide a subdirectory
to the default Git context.

Closes docker#460
Closes docker#528

README.md

@@ -43,6 +43,19 @@ By default, this action uses the [Git context](#git-context) so you don't need t
 done directly by buildkit. The git reference will be based on the [event that triggered your workflow](https://docs.github.com/en/actions/reference/events-that-trigger-workflows)
 and will result in the following context: `https://github.com/<owner>/<repo>.git#<ref>`.
 
+Beginning with BuildKit version `v0.9.0` ([Buildx](https://github.com/docker/buildx) `v0.7.0`) you can provide a subdirectory to the [Git context](#git-context) by using the magic variable `{{defaultContext}}`:
+
+```yaml
+      -
+        name: Build and push
+        id: docker_build
+        uses: docker/build-push-action@v2
+        with:
+          context: {{defaultContext}}:docker
+          push: true
+          tags: user/app:latest
+```
+
 Be careful because **any file mutation in the steps that precede the build step will be ignored, including processing of the `.dockerignore` file** since
 the context is based on the git reference. However, you can use the [Path context](#path-context) using the
 [`context` input](#inputs) alongside the [`actions/checkout`](https://github.com/actions/checkout/) action to remove

__tests__/context.test.ts

@@ -491,6 +491,24 @@ nproc=3`],
         '.'
       ]
     ],
+    [
+      15,
+      '0.7.0',
+      new Map<string, string>([
+        ['context', '{{defaultContext}}:docker'],
+        ['load', 'false'],
+        ['no-cache', 'false'],
+        ['push', 'false'],
+        ['pull', 'false'],
+      ]),
+      [
+        'buildx',
+        'build',
+        '--iidfile', '/tmp/.docker-build-push-jest/iidfile',
+        '--metadata-file', '/tmp/.docker-build-push-jest/metadata-file',
+        'https://github.com/docker/build-push-action.git#refs/heads/test-jest:docker'
+      ]
+    ],
   ])(
     '[%d] given %p with %p as inputs, returns %p',
     async (num: number, buildxVersion: string, inputs: Map<string, any>, expected: Array<string>) => {

src/context.ts

@@ -97,7 +97,11 @@ export async function getArgs(inputs: Inputs, defaultContext: string, buildxVers
   let args: Array<string> = ['buildx'];
   args.push.apply(args, await getBuildArgs(inputs, defaultContext, buildxVersion));
   args.push.apply(args, await getCommonArgs(inputs, buildxVersion));
+  if (inputs.context.startsWith('{{defaultContext}}') && buildx.satisfies(buildxVersion, '>=0.7.0')) {
+    inputs.context = inputs.context.replace('{{defaultContext}}', defaultContext);
+  }
   args.push(inputs.context);
+
   return args;
 }