Adapt the development Compose set-up to allow restricted company contexts

[jbaptperez]

Is your feature request related to a problem? Please describe.
The current Compose set-up for development is made for machines assumed to have an unrestricted access to the internet.

In several companies, accessing to internet (with different TLS certificates for TLS inspection) or remote repositories (Ubuntu/Debian PPAs, Node NPM or Python PyPI) is restricted and developers must use internal CA certificates or mirrors instead of official ones to get the necessary dependencies.
This happens in particular to build and even run (frontend) the Timesketch development image.

The actual Compose set-up does not allow the developer to adapt the image build process to his company restrictions, e.g. setting a different base image like a custom Ubuntu with company TLS certificates, setting their own internal mirror URLs or defining a specific configuration file for a package manager (npm or pip).

Describe the solution you'd like
Upgrade the development Compose set-up so that it allows a custom configuration using environment variables.

Custom environment variables should be defined in a .env file, read by Compose but not versioned in Git.
The default values should be obtained via a template file.
Custom values should be set in the .env file, and only in this file.

Describe alternatives you've considered
Modify the current Dockerfiles, Compose files, environment files, etc. to make them fit the company requirements, keeping them excluded from any Git versioning.
This would imply constant local file management, with the risk to include undesired files, by mistake, in commits related to contributions.
Moreover, every developers having company restrictions would maintain their own custom set-up.

Additional context
Such a feature would allow more contributors to join the project as they could contribute in a pure professional context.

Personally, I need such a feature in order to properly work on the issue #3219 in my company.

[jkppr]

Thanks for the suggested changes @jbaptperez . I'll have a look within the next week.

[jbaptperez]

@jkppr, I pushed a PR #3226 that I regularly rebase for a review.

Can you have a look?