Add Docker to Dependabot configuration
Add Docker to Dependabot configuration

We want to make sure we are using the latest version of our Docker base
images, let's use Dependabot to help us with that.

alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend