Suggestion: Add black[d]

Alternatively make a new image containing `black[d]`.
The black server is handy for using black without spawning processes for each run. Like for instance with a plugin like https://plugins.jetbrains.com/plugin/14321-blackconnect

Start by inspecting the repository's existing Docker image and build entry point, then compare how the current Black image is assembled with the requested blackd variant. Confirm the image can provide blackd for clients such as the linked BlackConnect plugin and document or test the resulting image usage.

Written by the indexing model from the issue text.