docker image: support configuration with environment variables

**Is your feature request related to a problem? Please describe.**

The APM docker container can not be configured with environment variables.
Environment variables have various advantages over the other available configuration mechanisms.
(Especially on Kubernetes)

**Describe the solution you'd like**

It would be nice if the apm container could be configured with environment variables, similar to the other elastic products.

**Describe alternatives you've considered**

* Volume mounting a custom configuration that uses custom configuration environment variables
* Has to be done by each user again.
* Has to be adapted for each option that the user wants to configure
* Makes discovery harder because for it to work users have to perform multiple steps before

* Volume-mounting a custom configuration file into the container
* A volume-mount is not as transparent as environment variables for the administrator
* The volume-mount contains credentials which should not be part of that
* All configuration options have to be managed as one entity

* Secret mounting a custom configuration file into the container
* A secret-mount is even less transparent
* Non-secret options are unnecessarily made less accessible
* All configuration options have to be managed as one entity

* Image with custom configuration
* Large overhead
* Even less transparent then the other options

Start by inspecting the APM Docker image and its existing configuration mechanisms, then compare how other Elastic products map environment variables to configuration options. Define the supported environment-variable behavior and verify it in a container-based test or reproduction. Done means the APM container can configure its options through environment variables without requiring a mounted custom configuration file.

Written by the indexing model from the issue text.