Handle AWS credentials

[crazy-max]

Fixes #19
Fixes #20

Allows to use aws-actions/configure-aws-credentials with our action for ECR registries. An example has been added in the README:

name: ci

on:
  push:
    branches: master

jobs:
  login:
    runs-on: ubuntu-latest
    steps:
      -
        name: Configure AWS Credentials
        uses: aws-actions/configure-aws-credentials@v1
        with:
          aws-access-key-id: ${{ secrets.AWS_ACCESS_KEY_ID }}
          aws-secret-access-key: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
          aws-region: <region>
      -
        name: Login to ECR
        uses: docker/login-action@v1
        with:
          registry: <aws-account-number>.dkr.ecr.<region>.amazonaws.com

Has been tested with AWS CLI v2:

And v1:

Signed-off-by: CrazyMax crazy-max@users.noreply.github.com