Add Dockerfile.test for local tests

[joanhey]

With this dockerfile is easier to make changes to the src code and then pass the tests locally.

Added to the readme:

Only to developers of ngx-php code !!

To build and run the Ubuntu 24.04 regression tests locally:

$ docker build --file Dockerfile.test --tag ngx-php-test .
$ docker run --rm ngx-php-test

To test locally all tests

docker run --rm ngx-php-test:local bash -c "export PATH=/src/nginx/sbin:/usr/bin:/bin; export TRAVIS_BUILD_DIR=/src; prove -r t"

To test only 1 test locally

docker run --rm ngx-php-test:local bash -c "export PATH=/src/nginx/sbin:/usr/bin:/bin; export TRAVIS_BUILD_DIR=/src; prove -v t/024-ngx_request_body.t"

We can also pass the arguments in the docker build

docker build --no-cache --build-arg PHP_VERSION=8.5 --build-arg NGINX_VERSION=1.31.4 ...