It's not like you have to maintain a fork, it's pretty minimal, all you need is a Dockerfile with what you want and build the container. Other than that you just keep bumping the version like you would the standard distribution.

For example to use rate limiting I just have a Dockerfile like this:

FROM caddy:2.9.1-builder AS builder

RUN xcaddy build --with github.com/mholt/caddy-ratelimit

FROM caddy:2.9.1

COPY --from=builder /usr/bin/caddy /usr/bin/caddy