I got it running (obviously after trying everything for hours and deciding to finally ask for help...) by switching to nvidia/cuda:11.7.1-base-ubuntu20.04 as my base image and adding the dependencies
libqt5multimediagsttools5 qtgstreamer-plugins-qt5
(libqt5multimediagsttools5 does not exist for Ubuntu 18.04) however various colleagues reported problems running importVideo on any newer Versions.
I've also encountered a different Problem which stopped me from using any 1.8.x versions (1.8.4 in particular):
chunk.exportRaster(os.path.join(results_dir, "tiles.zip"), format=Metashape.RasterFormat.RasterFormatTMS, image_format=Metashape.ImageFormat.ImageFormatPNG, source_data=Metashape.DataSource.OrthomosaicData, white_background=True, min_zoom_level=14, max_zoom_level=24, tile_width=256, tile_height=256, clip_to_boundary=True)
will somehow try to export zoom levels 0..7 (14..24 in 1.7.3) and fail with Vertical Datum out of range - not sure what that is about