build/contrib: Improve docker support.

[davecgh]

This significantly reworks the docker support to provide build a lightweight non-root distroless container image based on scratch. It employs a multi-stage build that downloads and builds the latest source code, compresses the resulting binaries, and then produces the final image based on scratch that only includes the Decred-specific binaries.

It must be noted that there are some still remaining TODO items in the documentation as well as the Dockerfile that will need to be handled by a future commit, but the changes are being submitted now to serve as a solid base for another contributor to finish up those aspects.

The new final image is only 10.2MB as compared to the existing version which is nearly 1GB.

The following is an overview of the changes:

• Removes existing Dockerfile and Dockerfile.alpine
• Introduces a new Dockerfile under contrib/docker with the following properties:
  • Runs as a non-root user
  • Uses a static UID:GID of 10000:10000
    • Note that using UIDs/GIDs below 10000 for container users is a security risk on several systems since a hypothetical attack which allows escalation outside of the container might otherwise coincide with an existing user's UID or existing group's GID which has additional permissions
  • The image is based on scratch image (aka completely empty) and only includes the Decred-specific binaries which means there is no shell or any other binaries available if an attacker were to somehow manage to find a remote execution vulnerability exploit in a Decred binary
• Introduces code to build an entrypoint for the image since it is based on scratch and thus has no shell for that purpose
• Adds contrib/docker/README.md
• Updates README.md in the main directory to account for changes
  • There is still outstanding work to be done here and thus has several TODOs
• Updates contrib/README.md to call out the new addition

This is work towards #2737.

[davecgh]

Updated for go1.17.1.