The default configuration in the docs is to add the following to the app.ini file:

[indexer]
; ...
REPO_INDEXER_ENABLED = true
REPO_INDEXER_PATH = indexers/repos.bleve
UPDATE_BUFFER_LEN = 20
MAX_FILE_SIZE = 1048576
REPO_INDEXER_INCLUDE =
REPO_INDEXER_EXCLUDE = resources/bin/**

After adding this to and restarting the app I get the following errors in the logs:

...er/issues/indexer.go:173:func2() [I] [644ad75f-12] PID 167: Initializing Issue Indexer: bleve
...exer/code/indexer.go:186:func3() [I] [644ad75f-14] PID: 167 Initializing Repository Indexer at: /home/git/gitea/indexers/repos.bleve
...xer/stats/indexer.go:38:populateRepoIndexer() [I] Populating the repo stats indexer with existing repositories
...ndexer/code/bleve.go:273:Close() [I] [644ad75f-14] PID: 167 Repository Indexer closed
...exer/code/indexer.go:200:func3() [F] [644ad75f-14] PID: 167 Unable to initialize the bleve Repository Indexer at path: /home/git/gitea/indexers/repos.bleve Error: mkdir /home/git/gitea/indexers: read-only file system
...exer/code/indexer.go:265:func4() [W] [644ad75f-14] Repository Indexer Initialization failed
341 INFO exited: gitea (exit status 1; not expected)

The main heart of the problem I believe is Unable to initialize the bleve Repository Indexer at path: /home/git/gitea/indexers/repos.bleve Error: mkdir /home/git/gitea/indexers: read-only file system.

In the start.sh file it has crudini --set "/run/gitea/app.ini" indexer ISSUE_INDEXER_PATH "/app/data/appdata/indexers/issues.bleve" which appears to configure the path for the indexer that indexs issues but maybe there is another setting that needs to be set correctly.