Allow adding custom external jars to classpath

[Trigtrig]

This is a draft showing the described changes to allow adding external jars to the classpath. I'm happy to add this functionality to the remaining images if you agree.

Description

Adds a new environment variable SE_EXTRA_LIBS which allows to add custom external jars to the classpath.

Motivation and Context

Selenium makes it possible to customize a node. While this works pretty well when running a node as shown in the documentation, it is not possible to achieve the same using the Docker images. Right now --node-implementation can be passed to the start-*.sh script using SE_OPTS, but there is no way to pass --ext to it.

Introducing a new environment variable representing --ext named SE_EXTRA_LIBS would allow to extend the classpath in a simple way. The desired jars could then be added to the Docker container in the user's preferred way:

• Create a customized image based on Seleniums images
• Mount the jars
• ...

This mechanism can be applied in the same way to all images.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.