Build restic at container build time

[jinnko]

What is the purpose of this change? What does it change?

Using docker's multi-stage builds we can build the restic/rest-server within a golang build environment then create a container for use (without the build environment) in a second build stage.

The advantages are:

1. Building the rest-server is predictable in a pristine environment each time.
2. Container builds ensure we get the latest rest-server every time.

Was the change discussed in an issue or in the forum before?

Not yet - will create an issue now.

https://github.com/restic/rest-server/issues/146

Checklist

NOTE: I will go through this checklist as soon as possible to satisfy all the requirements.

• [x] I have enabled maintainer edits for this PR
• [ ] I have added tests for all changes in this PR
• [x] I have added documentation for the changes (in the manual)
• [x] There's a new file in changelog/unreleased/ that describes the changes for our users (template here)
• [ ] I have run gofmt on the code in all commits
• [x] All commit messages are formatted in the same style as the other commits in the repo
• [x] I'm done, this Pull Request is ready for review

[jinnko]

Is there anything further you'd like to see in this PR before it's good to go?