OnContainerStartFunction and on_container_start are functions you can optionally implement in any container to get execution, per operation, when the container starts up. This is helpful when your container needs to do some housekeeping and prep an agent, c2 profile, or even eventing before anything else happens.
This function is one you can implement as part of the definition for your container (PayloadType, C2Profile, Eventing, etc).
What does it do?
This function gets an APIToken that is valid for 5 minutes and has the permissions of a spectator. This allows your container to query everything it needs, but not make any modifications.
When is it called?
This function is called when your container first comes online and syncs with Mythic. It's also called (as of Mythic 3.3.1-rc26) when anybody adds/removes/edits a file inside of your container through the UI. This allows you, the container developer, to be reactive to changes users make to files that might affect things like configurations.