Add docker-alpine example

[rina23q]

Proposed changes

This example contains Dockerfile for Alpine container and OpenRC service scripts for all services. The container will have those all together.

• mosquitto
• tedge
• tedge_mapper
• tedge_agent

Types of changes

What types of changes does your code introduce to thin-edge.io? Put an x in the boxes that apply

• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New example (non-breaking change which adds functionality)
• [ ] Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• [ ] Documentation Update (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• [x] I have read the CONTRIBUTING doc
• [x] I have signed the CLA (in all commits with git commit -s)
• [x] I have added necessary documentation (if appropriate)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[rina23q]

I will use squash and merge to clean up the commits.

[rina23q]

I did an update containing;

• Adjust Dockerfile to use the latest change by https://github.com/thin-edge/thin-edge.io/issues/761.
• Dockerfile does multi stage builds, so now docker build compiles all binaries.
• Update README.