The "Update Available" notification appears for services using a semantic version tag (like ghost:5.82.0) when a newer version is detected. If the available update is a minor or major version bump but your update preference is set to "patches only," it will show as available but won't be applied automatically. You can check your configured image tag and update preference under your service's Settings in the Source section. For reference, if a service is on the :latest tag (non-semantic), Railway skips the preference check and simply redeploys when a new image SHA is pushed to that tag. More details are in our Image Auto Updates docs.

That makes sense. So is the issue that :alpine tag is not treated the same way that :latest is? The docs seem to say that any non-semver tag will work the same way. The fact that the dashboard highlights that an update is available and that the dashboard allows me to enable auto-updates would suggest to me that it should work... but it is not.

That's correct. Any non-semver tag including :alpine is treated the same as :latest and checked by comparing the image digest, so auto-updates should fire when the digest behind :alpine changes. Since it has not over two months on a public image, this is behaving incorrectly on our side rather than a setting you've missed, and we're looking into why the digest change isn't being picked up here. In the meantime, triggering a redeploy from CI through our API after the image is rebuilt will pick up the new digest reliably.