The newer docker compose (vs docker-compose) allows you to set secrets in the build section. This is nice because if you do secrets at runtime then the file is readable by anyone that can get into the container by reading /run/secrets/<my_secret>.
Unfortunately, it appears that it's only possible to pass the secrets via either the environment or a file. Doing it via the environment doesn't seem like a great idea because someone on the box could read the /proc/<pid>/environment while the image is being built to snag the secrets. Doing it via a file on disk isn't good because then the secret is being stored on disk unencrypted.
It seems like the best way to do this would be with something like
docker swarm init
$(read -sp "Enter your secret: "; echo $REPLY) | docker secret create my_secret -
docker compose build --no-cache
docker swarm leave --force
Alas, it appears that Docker can't read from the swarm for build time secrets for some unknown reason.
What is the best way to do this? This seems to be a slight oversight, along the lines of docker secrete create not having a way to prompt for the value instead of having to resort to to hacks like above to keep the secret out of your bash history.