git: alternative github context and additional methods

[crazy-max]

follow-up docker/metadata-action#248

Adds alternative context to the GitHub one for Git.

@neilime PTAL. I keep you posted when we cut a new release of the actions-toolkit so you can rebase your PR on the metadata-action repo. You could then have smth like:

---
 src/context.ts | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/src/context.ts b/src/context.ts
index 3738b23..dd819c9 100644
--- a/src/context.ts
+++ b/src/context.ts
@@ -1,7 +1,11 @@
 import * as core from '@actions/core';
+import {Context} from '@actions/github/lib/context';
+import {Git} from '@docker/actions-toolkit/lib/git';
+import {GitHub} from '@docker/actions-toolkit/lib/github';
 import {Util} from '@docker/actions-toolkit/lib/util';
 
 export interface Inputs {
+  context: Context;
   images: string[];
   tags: string[];
   flavor: string[];
@@ -12,8 +16,9 @@ export interface Inputs {
   githubToken: string;
 }
 
-export function getInputs(): Inputs {
+export async function getInputs(): Promise<Inputs> {
   return {
+    context: await getContextInput('context'),
     images: Util.getInputList('images', {ignoreComma: true}),
     tags: Util.getInputList('tags', {ignoreComma: true}),
     flavor: Util.getInputList('flavor', {ignoreComma: true}),
@@ -24,3 +29,20 @@ export function getInputs(): Inputs {
     githubToken: core.getInput('github-token')
   };
 }
+
+export enum ContextSource {
+  workflow = 'workflow',
+  git = 'git'
+}
+
+async function getContextInput(name: string): Promise<Context> {
+  const input = core.getInput(name);
+  switch (input) {
+    case ContextSource.workflow:
+      return GitHub.context;
+    case ContextSource.git:
+      return await Git.context();
+    default:
+      throw new Error(`Invalid context: ${input}`);
+  }
+}

[neilime]

@crazy-max I've done the job on docker/metadata-action#248, waiting for you review