Fix docs for manual build/deployment

[sodic]

Version: 0.16.5

Our docs for manual server deployment don't mention the specific commands users must run to build the server.

The docs also don't mention that users must either:

1. Run docker build from the .wasp/build directory.
2. Explicitly specify the .wasp/build directory as the build context: docker build -t <app_name> .wasp/build/.

It's also probably useful to tell users how to run their server container. Basically, our docs for manual deployment should explain everything mentioned here: #1883 (comment).

Thanks for reporting @fbozic!