Linux AppImage build fails if run as root

Reported by @axel-kah

If you run a `briefcase create linux` build while logged in as root, it attemps to build the Docker container with HOST_UID and HOST_GID set to 0. This causes the `useradd` step to fail:
```
Step 12/16 : ARG HOST_UID
---> Running in 063a424d6a86
Removing intermediate container 063a424d6a86
---> 4657e96c180d
Step 13/16 : ARG HOST_GID
---> Running in a59336d3cdb7
Removing intermediate container a59336d3cdb7
---> 4fede95af7ee
Step 14/16 : RUN groupadd --gid $HOST_GID briefcase || true
---> Running in bb37131e7a73
groupadd: GID '0' already exists
Removing intermediate container bb37131e7a73
---> a94d6f409c76
Step 15/16 : RUN useradd --uid $HOST_UID --gid $HOST_GID brutus --home /home/brutus
---> Running in 55dc8b2957d8
useradd: UID 0 is not unique
The command '/bin/sh -c useradd --uid $HOST_UID --gid $HOST_GID brutus --home /home/brutus' returned a non-zero code: 4

Error building Docker container for helloworld.
```

This is because the default user in Docker is root, with UID 0.

We need to either:
a. Modify the docker container to be resilient to UIDs that already exist, or
b. Add error handling to prevent briefcase from being invoked with users that already exist.