Options to build firmware as a non-root user within a container ?

[phiblar]

I would like to build the firmware in a container. The requirements are as follows:

• The image for the container must be created from a Dockerfile.
• The firmware directory should be mounted in the container.
• The build process should run as a non-root user within the container.
• No changes should be necessary in the host system or in the firmware directory to achieve a successful build process.

I have tested several possibilities, but they have disadvantages:

• Building the firmware with a user inside the container requires using the U-flag and podman unshare.
• Using --userns=keep-id results in an increase in the image size, which necessitates modifying the storage.conf.

Are there any simple solutions for this particular case that I have overlooked?