Podman support

Tested with the Intel runner, `podman` seems to work fine with the following service:

```
[Unit]
Description=Buildkite agent %i
After=podman.service network-online.target
Requires=podman.service home.mount
Wants=network-online.target
PartOf=podman.service

StartLimitIntervalSec=0

[Service]
Type=simple
WorkingDirectory=/etc/buildkite/agents/%i
TimeoutStartSec=30min
ExecStartPre=/usr/bin/flock /tmp/buildkite.lock /usr/bin/git fetch
ExecStartPre=/usr/bin/flock /tmp/buildkite.lock /usr/bin/git reset --hard origin/main
ExecStartPre=/usr/bin/podman compose build
ExecStart=/usr/bin/podman compose up --force-recreate --exit-code-from buildkite

Restart=always
RestartSec=1s

StandardOutput=null

[Install]
WantedBy=multi-user.target
```