docker: create a dockerfile to setup plugin environment

Description

This Dockerfile pulls a wordpress docker image and copies the paystack plugin to the resulting container.

The Dockerfile would be used to startup an ECS container with wordpress running on it. This container would be used for tests.

The Dockerfile has been tested locally using docker-compose and used to run an ECS task. Link to the ECS loadbalancer- https://plugin-alb-1949830085.eu-west-2.elb.amazonaws.com/

[x] The resulting ECS container should have the Paystack payments plugin installed.

[ ] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

[x] My pull request addresses exactly one patch/feature.
[x] I have created a branch for this patch/feature.
[x] Each individual commit in the pull request is meaningful. (Please rebase, removing useless commits if not)
[ ] I have added tests to cover my changes.
[ ] All new and existing tests passed.
[x] My code follows the code style of this project.
[ ] My change requires change(s) to the environment.
[ ] My change requires change(s) to the database.
[ ] Other information.