Consider docker generation again

[philwebb]

Based on feedback we need to:

• Update the blog post about layered jars
• Check the documentation include the tools layer
• Consider if we want to offer a way to generate the docker file

[bclozel]

We've discussed this and came to the conclusion that:

• we've addressed the first two points already
• we don't think we can provide a generic/useful enough solution to generate docker files at the Spring Boot level

We're declining this issue as a result.