Allow using podman as an alternative building backend

**Is your feature request related to a problem? Please describe.**
Briefcase is currently using Docker as its building backend in Linux. This requires Docker to be installed and **callable in non-root environment**, which can be achieved by adding users to `docker` group. But as is mentioned in Docker's documentation, this operation **grants privileges equivalent to the root user to normal user**. Although Docker does provide an experimental rootless mode, `podman` seems to be more stable, and is adopted by distributions like Fedora as the replacement to Docker.

(However most developers don't use a multi-user machine so this issue seems to be superfluous lolol)

**Describe the solution you'd like**
On the website of podman, it says `alias docker=podman`. I hope implementing it won't be difficult.

**Describe alternatives you've considered**
[Rootless Docker](https://docs.docker.com/engine/security/rootless/)

No file or test is named in the issue. Start by locating Briefcase’s Linux build backend and the existing Docker command path; compare Podman’s compatibility there, with done defined as supporting Podman as an alternative without breaking Docker builds.

Written by the indexing model from the issue text.