Release installer DMG does not display the background image

The opened DMG installer window is not displaying the background image, which has the Jalview logo tiled background, the Jalview name (in correct font), and the beta-sheet arrow indicating how to install the app (also a git-commit and date in grey).

This is significant loss of user experience so worth pursuing.

The process by which a background image is added to the DMG file is a bit convoluted, specific and manual (as described in doc/README-DMG_creation.md). This is how install4j recommends adding the background image (it cannot do it directly itself).
The method to create the DS_Store file described here must be performed on a mac, but the resulting DS_Store file can be used on any machine building the DMG with install4j.

There have been some attempts to automate DS_Store creation, even using tools that run on linux. These could be explored further.

See https://issues.jalview.org/browse/JAL-3393
and https://github.com/create-dmg/create-dmg

Also (both work on macOS only):
https://github.com/LinusU/node-appdmg/tree/master
https://pypi.org/project/dmgbuild/