Only if you're not using it for commercial stuff. If you're using it for commercial stuff you have to pay the $25/month
"If your company uses official Caddy binaries internally, in production, or distributes Caddy, a commercial license is required."
"If I build Caddy from source, which license applies?" "The source code is Apache 2.0 licensed."
I.e. you get a byte-identical binary file if you build it yourself, but you don't have to pay the $25/mo. Even though you end up at exactly the same place.
Something like http://ansuz.sooke.bc.ca/entry/23 "What colour are your bits?" I suppose.