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

[mamekoro]

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 <environment_variable>=/path_to_runtime in the Dockerfile.

[TheAssassin]

I think this is a usage issue rather than a Docker or appimagetool issue.

If you need a way to build an AppImage allowing you to configure the build with environment variables, check out linuxdeploy-plugin-appimage, a wrapper around appimagetool.

[probonopd]

@TheAssassin would having the env hurt though?