There is an apparently Node.js implementation [2] of something that can publish a given torrent to a mutable address, and also retrieve a mutable torrent from a given address. I do not know how well this is implemented in the usual clients, but if you want it in your docker, you might want to talk to libtorrent directly, and implementing BEP 46 yourself should not be hard with the things the library has to offer. A benefit could be that depending on how you handle it, you might be able to store the tarballs docker images seem to be in their unpacked form, and just keep some metadata about what the header(s) of the tarball were, along with some file offsets. This way you would be relieved of the unnecessary storage burden, and able to possibly use many more of your servers to seed at least part of the images, e.g. maybe only the parts that are not mutated when the software is running. E.g., download once, unpack, only offer to seed those files/pieces that did not get modified in the meantime, without trying to re-download the "broken" data.
[0]: https://www.libtorrent.org/dht_store.html [1]: http://www.bittorrent.org/beps/bep_0046.html [2]: https://github.com/lmatteis/dmt