Add stack option to change stack if there is any need

- Whenever we create a heroku app, the stack of the app sets as heroku-20.
  More about stacks [0].
- For Building docker images with heroku yml, We need to set the stack of the app to container.
  So lets have the ability to change the stack through our action.

reference:
[0] - https://devcenter.heroku.com/articles/stack

README.md

@@ -97,6 +97,7 @@ The action comes with additional options that you can use to configure your proj
 | env_file                    | false    | path to an env file (with respect to appdir)                                                                                                                                                        | /.env                                                 |
 | justlogin                   | false    | Set to true if you want the action to just login to Heroku and nothing else                                                                                                                         | true or false                                         |
 | region                      | false    | The region in which you would like to deploy a server                                                                                                                                               | eu or dublin                                          |
+| stack                      | false    | Set stack of your heroku app if you need to change. Default: heroku-20                                                                                       | container                                          |
 | team                        | false    | If deploying to an organization, then specify the name of the team or organization here                                                                                                             | team-xyz                                              |
 
 ## Examples
@@ -281,6 +282,37 @@ jobs:
 
 Though this is also possible to do with GitHub Actions, click [here](https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#on) for more information
 
+### Set stack for your app
+
+In some cases, you need to change the default stack - heroku-20.
+For example, If you are building docker images with heroku yml, you need to change the stack to container.
+You can use the **stack** option to change stack for your app.
+
+_.github/workflows/main.yml_
+
+```yaml
+name: Deploy
+
+on:
+  push:
+    branches:
+      - master # Changing the branch here would also work
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: akhileshns/heroku-deploy@v3.11.10 # This is the action
+        with:
+          heroku_api_key: ${{secrets.HEROKU_API_KEY}}
+          heroku_app_name: "YOUR APP's NAME" #Must be unique in Heroku
+          heroku_email: "YOUR EMAIL"
+          stack: "container"
+```
+
+Though this is also possible to do with GitHub Actions, click [here](https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#on) for more information
+
 ## Health Check
 
 Sometimes you will run into issues where the action has successfully deployed the project but because of some error in code or the like, the Heroku App crashes or fails to launch. To counter this, you can setup a healthcheck in the action:

action.yml

@@ -75,6 +75,10 @@ inputs:
     description: "The region in which you would like to deploy a server"
     required: false
     default: ""
+  stack:
+    description: "Set stack of your heroku app if you need to change.Default : heroku-20"
+    required: false
+    default: ""
   team:
     description: "If deploying to an organization, then specify the name of the team or organization here"
     required: false

index.js

@@ -28,6 +28,7 @@ const addRemote = ({ app_name, dontautocreate, buildpack, region, team }) => {
         app_name +
         (buildpack ? " --buildpack " + buildpack : "") +
         (region ? " --region " + region : "") +
+        (stack ? " --stack " + stack : "") +
         (team ? " --team " + team : "")
     );
   }
@@ -148,6 +149,7 @@ let heroku = {
   env_file: core.getInput("env_file"),
   justlogin: core.getInput("justlogin") === "false" ? false : true,
   region: core.getInput("region"),
+  stack: core.getInput("stack"),
   team: core.getInput("team"),
 };