Dangerzone may fail to run with user ID other than 1000

[apyrgio]

When Dangerzone invokes Podman, it passes a --userns keep-id argument. This invocation basically binds the user ID in the host (typically 1000) with the exact same user ID in the container (again 1000). This way, the container user (see Podman's -u dangerzone flag) can write to the mounted temp dir in the container (i.e., /tmp/dangerzone) which is owned by the user in the host.

If Dangerzone runs from a user in the host whose ID is != 1000 (e.g., 1001), then we have a problem, because the container user will still have user ID 1000, and thus --userns keep-id will not work.

Related issues: #620 Will be resolved by: #443, #625

[apyrgio]

We had another user who was bitten by this (https://github.com/freedomofpress/dangerzone/issues/781). A workaround in their case was to create a user account with UID 1000, and run Dangerzone as that user with sudo -iu <user> dangerzone.

Once we merge #625, this workaround will no longer be necessary.