Make selenium component images configurable

[Zerpet]

Proposed Changes

This PR makes all selenium component images configurable. Some components were already configurable, but the variable declaration was inconsistent i.e. some components declared the image variable at the top of the script, others, inside an init function. This PR makes the image variable declaration consistent.

This PR also makes it easy to override them in CI. For example, to use a proxy to access DockerHub or GHCR.

Types of Changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

Further comments

We should backport this to 4.2 and 4.1. This can wait until 4.2.0 release.