add changelog for #678

Author: Emilgardis

Diff for: CHANGELOG.md

@@ -5,6 +5,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+- #678 - Add `target.{target}.dockerfile` and optional `target.{target}.context` to invoke `docker/podman build` before using an image. 
 - #665 - when not using [env.volumes](https://github.com/cross-rs/cross#mounting-volumes-into-the-build-environment), mount project in /project
 - #624 - Add `build.default-target`
 - #670 - Use serde for deserialization of Cross.toml