in your Dockerfile, so your application is running as the predefined app user which has UID 64198.

For you to be able to create files in the mounted directory, UID 64198 needs to be able to create files on the host in the ./testdata directory.

You can do that by giving public write access on the host using chmod o+w ./testdata.

If that's too permissive, you can create a user on the host with UID 64198 and give that user group access to the directory.

in your Dockerfile, so your application is running as the predefined app user which has UID 64198.

For you to be able to create files in the mounted directory, UID 64198 needs to be able to create files on the host in the ./testdata directory.

You can do that by giving public write access on the host using chmod o+w ./testdata.

If that's too permissive, you can create a user on the host with UID 64198 and give that user group access to the directory.