Proxy is supplied as a pre-built docker image

We've proposed shipping YARP in three formats:
- A library - https://www.nuget.org/packages/Microsoft.ReverseProxy/
- A template - https://github.com/microsoft/reverse-proxy/issues/77
- A pre-built application exe #261
- A docker image (this issue)

The pre-built app should:
- Read from a config file
- Cover basic scenarios
- Support a variety of platforms

# Scenarios
- Config based routes, clusters, & backends
- Remote route config? E.g. load from a database, service fabric, k8?
- Dashboards? https://github.com/microsoft/reverse-proxy/issues/123
- Health checks for the proxy itself.
- Static files? https://github.com/microsoft/reverse-proxy/issues/187
- Auth?
- Multi-instance?
- Data protection (Session Affinity)
- Single-file exe with linking: https://github.com/microsoft/reverse-proxy/issues/7
- Config UI? https://github.com/microsoft/reverse-proxy/issues/184

# Config
The library already supports loading routes from a config file. However, an app will need to expose additional config for the scenarios described above.

# Platforms
- Windows, Mac, Linux
- Builds for both runtime dependent and standalone (servicing?)

# Acquisition
- Microsoft download center?