Fix magnet-certbot build context for Docker

The flyctl deploy command couldn't find renew.sh because the build
context wasn't explicitly set. When running with --config from the repo
root, the context defaults to cwd rather than the fly.toml directory.

Adding context = "." makes the build context explicitly the same
directory as the fly.toml and Dockerfile.

Author: perigrin

servers/magnet-certbot/fly.toml

@@ -6,6 +6,7 @@ primary_region = "ord"
 
 [build]
   dockerfile = "Dockerfile"
+  context = "."
 
 [http_service]
   internal_port = 80