Feature request: allow HTTP request customizations

[DDtKey]

This issue comes from #395 (comment)

It would be nice to be able to customize request to docker.
There is a need to at least configure the headers for the testcontainers crate: testcontainers/testcontainers-rs#576

But in general it would be more flexible to have some general way of doing this. For example, a callback function that accepts a mutable request entity (or builder). Hypothetically, this allows people to create an invalid request - but in this case it is already their responsibility if such an API is used.

[fussybeaver]

Closed due to implementation done in #459