One cannot use
pihole checkout ftl fix/something
in docker. I request this to be changed.
I'm willing to work on this myself but my knowledge is limited when it comes to docker. So I need a pointer to why this does not work at the moment. In my naive view, it should just replace the binary and restart the service.
The change does not have to be permanent, it does not have to survive container restarts.
That's not how things work in the land of Docker. You don't make those kinds of changes inside the container.
I don't think we can even do a docker image per branch since each repo has their own branches and that would be an exponential number of images to provide.
For what it's worth, one can! I did so yesterday to try a tweak to the web interface on my non-dev device. But, as Dan quite rightly pointed out, one probably shouldn't.
You just need to jump into the container first with docker exec -it pihole bash or just prefix pihole commands with docker exec <container_name> pihole <pihole command>
If you're having issues with the above please share your commands and output of what you ran.
For what it's worth, that command wouldn't work anyways, 
Edited/fixed the command in the first message. Can I make this change permanent? I was thinking about getting a docker image for playing around with and I want a special FTL branch, say new/superclients.
Can I run a command and commit a "new" container where the FTL was replaced?
Sorry if this sounds ignorant or naive, I find it still a bit hard to use the right vocabulary with docker and its ecosystem.