feat: allows to specify context to fetch git data

Signed-off-by: Emilien Escalle <emilien.escalle@escemi.com>

src/context.ts

@@ -1,7 +1,11 @@
 import * as core from '@actions/core';
+import {Context} from '@actions/github/lib/context';
 import {Util} from '@docker/actions-toolkit/lib/util';
+import {Git} from '@docker/actions-toolkit/lib/git';
+import {GitHub} from '@docker/actions-toolkit/lib/github';
 
 export interface Inputs {
+  context: ContextSource;
   images: string[];
   tags: string[];
   flavor: string[];
@@ -14,6 +18,7 @@ export interface Inputs {
 
 export function getInputs(): Inputs {
   return {
+    context: (core.getInput('context') || ContextSource.workflow) as ContextSource,
     images: Util.getInputList('images', {ignoreComma: true}),
     tags: Util.getInputList('tags', {ignoreComma: true}),
     flavor: Util.getInputList('flavor', {ignoreComma: true}),
@@ -24,3 +29,45 @@ export function getInputs(): Inputs {
     githubToken: core.getInput('github-token')
   };
 }
+
+export enum ContextSource {
+  workflow = 'workflow',
+  git = 'git'
+}
+
+export async function getContext(source: ContextSource): Promise<Context> {
+  switch (source) {
+    case ContextSource.workflow:
+      return getContextFromWorkflow();
+    case ContextSource.git:
+      return await getContextFromGit();
+    default:
+      throw new Error(`Invalid context source: ${source}`);
+  }
+}
+
+function getContextFromWorkflow(): Context {
+  const context = GitHub.context;
+
+  // Needs to override Git reference with pr ref instead of upstream branch ref
+  // for pull_request_target event
+  // https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target
+  if (/pull_request_target/.test(context.eventName)) {
+    context.ref = `refs/pull/${context.payload.number}/merge`;
+  }
+
+  // DOCKER_METADATA_PR_HEAD_SHA env var can be used to set associated head
+  // SHA instead of commit SHA that triggered the workflow on pull request
+  // event.
+  if (/true/i.test(process.env.DOCKER_METADATA_PR_HEAD_SHA || '')) {
+    if ((/pull_request/.test(context.eventName) || /pull_request_target/.test(context.eventName)) && context.payload?.pull_request?.head?.sha != undefined) {
+      context.sha = context.payload.pull_request.head.sha;
+    }
+  }
+
+  return context;
+}
+
+async function getContextFromGit(): Promise<Context> {
+  return await Git.context();
+}