Make `--runtime-file` specifiable via an environment variable

When building a project that produces an AppImage file (e.g., KeePassXC) in a Dockerfile, I need to parse and edit the build scripts of the project to pass `--runtime-file ./path_to_downloaded_runtime_file` to the `appimagetool` command.

Could you consider making `--runtime-file` specifiable via an environment variable? If that becomes possible, I would only need to specify `ENV =/path_to_runtime` in the Dockerfile.