Provide option to use pre-downloaded Bedrock server zip

I ran it with the `DEBUG=true` environment variable set, but [un]fortunately this time the download did not fail and the server container was deployed correctly. What might be helpful for the future is an environment variable (or instructions) to load the zip directly from `/data` instead of downloading it, for people on flaky connections.

_Originally posted by @chr15m in https://github.com/itzg/docker-minecraft-bedrock-server/issues/115#issuecomment-745030466_

Start by reading the container startup logic that downloads the Bedrock server zip and handling around `/data`, using the issue's `DEBUG=true` context to trace the path. Done means an environment variable or documented option lets the container use a pre-downloaded zip from `/data` instead of downloading it.

Written by the indexing model from the issue text.