Refine PHP docker image and mark as GA

[ford-at-aws]

As an AWS customer, I want to be able to rely on the Docker image found in the PHP language repository to contain all the necessary examples for PHP, including pre-compiled dependencies so that I can effectively run example code in a Docker environment.

I know this is done when:

• the Dockerfile itself contains correct steps for resolving dependencies and is built successfully
• the resulting Docker image is successfully pushed to ECR via the existing DevOps processes
• the container that is created from that image runs some subset of example code (e.g. the Lambda MVP) successfully by exec'ing in, adding AWS credentials, and invoking it via terminal session
• usage steps explaining which examples can be run and how are documented in the README.md#docker-image section.

Adding credentials to your docker container

Never add credentials to a Dockerfile or the resulting image. Instead, add them at runtime using the following command:
docker run -it --volume ~/.aws/credentials:/root/.aws/credentials IMAGE_NAME

You can find IMAGE_NAME by running docker images and copying the first 4 digits of the random Id that's created when you run docker build .