Document How to Add Non-Scratch Dependencies

MX Bluesky uses dependencies such as `opencv-python`, which requires various libraries be acquired via `apt install`. There is no
documentation on the preferred method to add such dependencies.

Currently MX are maintaining an image based on "standard" BlueAPI, with an additional layer on top which installs all additional requirements.

We need to agree on the preferred method, and document this process.

See [relevant Slack thread](https://diamondlightsource.slack.com/archives/C051NDJH6BZ/p1765386776965259).