Add image_aliases and hook script support

[rnorth]

This is intended to provide a crude solution to testcontainers/testcontainers-java#3099 for teams using Drone CI.

It allows users to specify a set of images that should be pulled and retagged, and exposes a hook script mechanism so that more custom implementations can be developed.

Note that I believe testcontainers/testcontainers-java#3102 is by far the better solution, but I believe this provides another option for Drone users who cannot use that approach.

Example usage from README:

pipeline:
  build:
    image: quay.io/testcontainers/dind-drone-plugin
    build_image: openjdk:8-jdk-alpine
    cmd: ./gradlew clean check --info

    # Not mandatory; sets up image name 'aliases' by pulling from one registry and tagging
    #  as a different name. Intended as a simplistic mechanism for using a private registry 
    #  rather than Docker Hub for a known set of images. Accepts a dictionary of
    #  private registry image name to the Docker Hub image that it is a substitute for.
    #  Note that all images are pulled synchronously before the build starts, so this is
    #  inefficient if any unnecessary images are listed.
    image_aliases:
      someregistry.com/redis:4.0.6: redis:4.0.6

[gjtempleton]

It's been a while since I did drone plugin dev work, does Drone inject the dict variables as JSON formatted text for the env var? (I did try digging through the Drone docs and couldn't see any mention of how this is handled.)

[gjtempleton]

Managed to demo the answer to my own question, it does indeed.

[rnorth]

Yeah, it's weirdly inconsistent about it though:

• for a single value drone parameter, it comes through as a single plain env var
• for a list parameter, it comes through comma-separated (with no usable escaping AFAICT 😱)
• for a dict parameter, it comes as a JSON dict

I honestly have no idea why, but it is what it is 🤷‍♂️

[gjtempleton]

LGTM

[rnorth]

Thanks @gjtempleton