Reported in WIkibase telegram group.
```
My wikibase instance throws: [5e5fbdd67bad75f7880ce5c3] 2020-02-05 13:18:13: Fatal exception of type "Error"
```
When @Addshore tried this locally the full stacktrace shows:
```
[77bf49be7604c080797560d0] / Error from line 1040 of /var/www/html/extensions/Wikibase/repo/includes/WikibaseRepo.php: Class 'Wikibase\DataModel\Services\EntityId\EntityIdComposer' not found
Backtrace:
#0 /var/www/html/extensions/Wikibase/repo/includes/WikibaseRepo.php(2262): Wikibase\Repo\WikibaseRepo->getEntityIdComposer()
#1 /var/www/html/extensions/Wikibase/repo/includes/WikibaseRepo.php(2253): Wikibase\Repo\WikibaseRepo->newMultipleRepositoryAwareWikibaseServices()
#2 /var/www/html/extensions/Wikibase/repo/includes/WikibaseRepo.php(1973): Wikibase\Repo\WikibaseRepo->getWikibaseServices()
#3 /var/www/html/extensions/Wikibase/repo/RepoHooks.php(121): Wikibase\Repo\WikibaseRepo->getEntityNamespaceLookup()
#4 /var/www/html/includes/Hooks.php(174): Wikibase\RepoHooks::onSetupAfterCache()
#5 /var/www/html/includes/Hooks.php(202): Hooks::callHook(string, array, array, NULL)
#6 /var/www/html/includes/Setup.php(782): Hooks::run(string)
#7 /var/www/html/includes/WebStart.php(77): require_once(string)
#8 /var/www/html/index.php(39): require(string)
#9 {main}
```
Prior to this in the install logs for the container things already broke in a similar way:
```
wikibase_1 | Creating main page with default content
wikibase_1 | Main page already exists, skipping
wikibase_1 | done
wikibase_1 | Database was successfully set up
wikibase_1 | MediaWiki has been successfully installed. You can now visit </wiki> to view your wiki. If you have questions, check out our frequently asked questions list: <https://www.mediawiki.org/wiki/Special:MyLanguage/Manual:FAQ> or use one of the support forums linked on that page.
wikibase_1 | Done!
wikibase_1 | [918300def4f7f404996337a2] [no req] Error from line 1040 of /var/www/html/extensions/Wikibase/repo/includes/WikibaseRepo.php: Class 'Wikibase\DataModel\Services\EntityId\EntityIdComposer' not found
wikibase_1 | Backtrace:
wikibase_1 | #0 /var/www/html/extensions/Wikibase/repo/includes/WikibaseRepo.php(2262): Wikibase\Repo\WikibaseRepo->getEntityIdComposer()
wikibase_1 | #1 /var/www/html/extensions/Wikibase/repo/includes/WikibaseRepo.php(2253): Wikibase\Repo\WikibaseRepo->newMultipleRepositoryAwareWikibaseServices()
wikibase_1 | #2 /var/www/html/extensions/Wikibase/repo/includes/WikibaseRepo.php(1973): Wikibase\Repo\WikibaseRepo->getWikibaseServices()
wikibase_1 | #3 /var/www/html/extensions/Wikibase/repo/RepoHooks.php(121): Wikibase\Repo\WikibaseRepo->getEntityNamespaceLookup()
wikibase_1 | #4 /var/www/html/includes/Hooks.php(174): Wikibase\RepoHooks::onSetupAfterCache()
wikibase_1 | #5 /var/www/html/includes/Hooks.php(202): Hooks::callHook(string, array, array, NULL)
wikibase_1 | #6 /var/www/html/includes/Setup.php(782): Hooks::run(string)
wikibase_1 | #7 /var/www/html/maintenance/doMaintenance.php(81): require_once(string)
wikibase_1 | #8 /var/www/html/maintenance/update.php(275): require_once(string)
wikibase_1 | #9 {main}
wikibase_1 | wait-for-it.sh: waiting 300 seconds for mysql.svc:3306
wikibase_1 | wait-for-it.sh: mysql.svc:3306 is available after 0 seconds
```
It looks like code that needs composer packages is running before they are ever loaded by mediawiki.
This is fallout from using the composer merge plugin in the images, but should also happen to people using the merge plugin not in the containers..