tests/docker: Specify --userns keep-id for Podman

tests/docker: Specify --userns keep-id for Podman

Previously we are always specifying -u $(UID) to match the UID in the
container with one outside. This causes a problem with rootless Podman.

Rootless Podman remaps user IDs in the container to ones controllable
for the current user outside. The -u option instructs Podman to use
a specified UID in the container but does not affect the UID remapping.
Therefore, the UID in the container can be remapped to some other UID
outside the container. This can make the access to bind-mounted volumes
fail because the remapped UID mismatches with the owner of the
directories.

Replace -u $(UID) with --userns keep-id, which fixes the UID remapping.
This change is limited to Podman because Docker does not support
--userns keep-id.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Message-Id: <20240626-podman-v1-1-f8c8daf2bb0a@daynix.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240705084047.857176-6-alex.bennee@linaro.org>