Add Docker image build CI

[Chun-Sheng, Li]

As title, I think it should be nice to have the Docker image building with CI.

IMHO, I think we can use GitHub Actions or Travis CI to accomplish this issue.

@DragonBe, do you have any suggestion about this?

Once this issue is accepted, I think I would be happy to work on this 😄 .

[M van Dam]

Question I have is: what would you do with the container?

Option 1: run phpunit in the container
Option 2: run a web app for validating one or more VAT ID's (asynchronously)

For option 1 I run the following command to execute it with the latest PHP 7.4 stack

docker run -it --rm --name vies-test -v "$PWD":/usr/src/myapp -w /usr/src/myapp dragonbe/vies-php:7.4-cli vendor/bin/phpunit --no-coverage

This is what I also use in my internal Jenkins pipeline to ensure all is working, even in a clean environment.

For option 2, I already have a project called "dragonbe/vies-web" where you can run a web interface for this service. Maybe there we could add a docker container to build and run it everywhere?

[Chun-Sheng, Li]

@DragonBe, thanks for your reply.

Option 1: This may have some descriptions on README to let developers know how to run unit tests for this project with PHPUnit.

Option 2: Yes. It may be convenient and easy for developers to run vies-web with Docker container everywhere.

[M van Dam]

Added a stand-alone container in #131