Powerbuilding with Fly

Summary

Powerbuilding with Fly explains how flyctl controls Docker-image builds when deploying applications to Fly infrastructure. It covers selecting the application configuration with --config, choosing an alternate Dockerfile with --dockerfile, and passing build-time values through --build-args. Using NGINX_PORT as an example, the post shows how Docker ARG can receive a default or command-line value and how ENV=${NGINX_PORT} carries that value into the running image; flyctl deploy --build-args NGINX_PORT=4000 overrides 8080. Build arguments are intended for non-sensitive data because values are baked into the image and may be retrievable, while Fly Secrets are recommended for sensitive application data and are injected as environment variables when the image starts.

Takeaways

1. 1

   flyctl assumes fly.toml in the current directory, while --config selects another configuration file and --dockerfile selects an alternate Dockerfile when building without a builder.

2. 2

   The pattern ARG NGINX_PORT=8080 followed by ENV NGINX_PORT=${NGINX_PORT} lets a build argument become a runtime environment variable; --build-args NGINX_PORT=4000 overrides the default.

3. 3

   Build arguments are unsuitable for credentials because their values are baked into the image and may be retrievable; Fly Secrets are injected when the image starts.