List of steps to reproduce (step by step, including full links if applicable):

• Install mwcli.
• Set up all containers. ( mw dev mediawiki create && mw dev mysql create)
• Clone or download StructuredDiscussion in the 'extensions' directory.
• (If LocalSettings.php exists, then) Append wfLoadExtension( 'Flow' ); to it.
• Install MediaWiki by executing mw dev mediawiki install --dbtype mysql. (Flow does not support sqlite)
• (If LocalSettings.php did not exist, then) Append wfLoadExtension( 'Flow' ); to it and rerun update.php.
• Append $wgNamespaceContentModels[NS_TALK] = 'flow-board'; to the LocalSettings.php.
• Visit http://default.mediawiki.mwdd.localhost:8080/w/index.php?title=Talk:Main_Page

What happens?:

A PHP warning displays.

Warning: fopen(/var/www/html/w/images/docker/default/tmp/mw-GlobalIdGenerator-UID-88): failed to open stream: No such file or directory in /var/www/html/w/includes/libs/uuid/GlobalIdGenerator.php on line 437

The directory the path indicates doesn't seem to be exist.

$ mwdev dev mediawiki exec ls /var/www/html/w/images/docker/default
cache

What should have happened instead?:

An empty Flow board should have been shown when visiting the talk page.

Software version:

• mwcli: d9c1850
• MediaWiki: 61b534829a
• Flow: 047e88e0

Other information, etc:
The document on mediawiki.org says:

You must run "update.php" as the web server user (depends on configuration, but could be e.g. www-data). Otherwise, it can leave temporary /tmp/mw-UIDGeneratorUUID-UID-xx files that MediaWiki may be unable to open (task T55791). If you get a "Permission denied" error upon first visiting a Flow board you have to delete these files. Also, update.php will create {{FlowMention}} to support Flow's mention feature in VE (either @ or a toolbar menu provides a convenient auto-complete interface for mentions).