Use Docker for all unit testing

[TomHAnderson]

Feature Request

Q	A
New Feature	yes
RFC	no
BC Break	no

RFC

https://discourse.laminas.dev/t/rfc-use-docker-for-unit-testing/1774

Summary

Testing of this repository currently requires an installation of Mongo. The developer should not be expected to maintain such an installation on their machine in order to run unit tests. I am proposing all unit testing be moved to Docker. This may be done in two stages.

During this change the minimum version of PHP will be moved to 7.3 for reasons including composer no longer support PHP 5.6.

Stages

Stage 1 is required but stage 2 is only necessary if those involved want to proceed.

1. Change running of phpunit to use a docker image. Maintain Travis CI testing as-is.
2. Change Travis CI testing to use Docker and build images in the same process as the phpunit image.

Note

This is not the first time I've made this proposal. I am proposing it again so the [new] Laminas requirements for new features can be followed and by doing so I expect to bring in the involvement of the maintainer(s) of this repository.