Improve the README.md

The changes are mostly cosmetical in that they are intended to improve the readability of the README without changing the content, except for changing the wording at some places.

The only "real" exception is in the section "Local testing in Docker", where the separate `docker build` invocations have been replaced with a single `./build_all.bash` invocation.
These instructions are not specific to EC2. That's just where we are testing for ease.  Some deployments are expected on EC2. But there are many use cases where this will be deployed inside a corporate firewall.