Apply COPY vs ADD best practices

Should only use `ADD` for URLs (and even then, `curl` or `wget` are preferred, for numerous reasons, even within this repo) and when extracting archive contents as part of the `ADD` operation; otherwise `COPY` is clearer and (slightly) more efficient.

nginx/Dockerfile

@@ -2,7 +2,7 @@ FROM nginx:alpine
 
 LABEL maintainer="Mahmoud Zalt <mahmoud@zalt.me>"
 
-ADD nginx.conf /etc/nginx/
+COPY nginx.conf /etc/nginx/
 
 # If you're in China, or you need to change sources, will be set CHANGE_SOURCE to true in .env.