Use jemalloc in the docker image

[Half-Shot]

I couldn't find an issue for this, but I think it's been brought up in conversation every now and then. I don't know if there is a reason we don't do this (ARM, maybe?).

If we don't want to do this, that's fine to and it means I can shut down the upstream conversation :)

[clokep]

You're probably thinking of #8553. I think that has the most up-to-date conversation in it.