Move static binaries to dist/ for easier publishing

This will make it easier to publish them to Github or Gitea releases later.

To avoid making the Makefile super complex, this patch also makes use of
variables to simplify the Makefile as well.

Dockerfile

@@ -20,7 +20,7 @@ RUN chmod -R 755 /app/scripts
 
 # Copy minitor in
 ARG ARCH=amd64
-COPY ./minitor-linux-${ARCH} ./minitor
+COPY ./dist/minitor-linux-${ARCH} ./minitor
 
 # Drop to non-root user
 USER minitor