Wrong file owners when syncing project to container

[halloei]

When developing my application, the default user of the container is root, but I am also creating a custom user (appuser) that I switch to when attaching to the container.
The issue is that the files being synced into the container are owned by root:root, so that the application is not able to create/modify/delete files within the container.

I have a startup script that basically looks like this:

#!/usr/bin/env bash

devspace deploy

# Initial file sync into the container
devspace sync --initial-sync preferLocal --no-watch --pod <pod-name> --container app

devspace dev --skip-build --skip-deploy

Afterward, the files are constantly being synced (bidirectional):
devspace.yml (excerpt):

deployments:
  app:
    namespace: example-namespace
    helm:
      chart:
        name: helm-chart
      valuesFiles:
        - helm-chart/values.yaml

dev:
  app:
    sync:
      - path: .:/app
        initialSync: preferRemote

The Dockerfile:

FROM php:8.3

RUN useradd -u <id-of-the-host-user> -g <id-of-the-host-group> appuser

There is no USER appuser, as I want to keep root as default user, but I am switching to appuser when entering the container in the .bashrc file.
I tried USER appuser anyway of course, but then the sync fails:

start_dev: initial sync: upstream: apply changes: apply creates: upload archive: upload send: rpc error: code = Unknown desc = untar all: decompress: error creating /app/vendor: mkdir /app/vendor: permission denied

What do I need to do in order to keep the file owners of my host when syncing to the container?