When using the Matomo container created by #238, I'm experiencing a few missing images

A Matomo issue Morpheous icons missing on dashboard #16553 indicates when using the GitHub release, the resulting download maybe missing any Git Submodules within the repository. The recommendation in the GitHub issue is to "Please use the releases published on https://builds.matomo.org/".
The current Matomo Dockerfile uses the GitHub release approach and misses icons included within the plugins/Morpheus/icons/dist/flags/xx.png.
The impact:
- largely appearance; analytics continue to be recorded and viewable.