Add parameter to add additional docker tags

[jameslnewell]

This PR adds a tags: <list> parameter to in order to enable tagging of images with additional docker tags.

This is useful if you want to be able to reference the image later in the pipeline e.g. from a docker-compose file

Example usage:

# pipeline.yml

steps:
  - command: echo wow
    plugins:
      - seek-oss/docker-ecr-cache#v2.0.0:
          ecr-name: <repository>
          tags:
            - build-${BUILDKITE_BUILD_NUMBER}

# docker-compose.yml

services:
  test:
    image: <registry>/<repository>:build-${BUILDKITE_BUILD_NUMBER}
    command: npm run test