Maintain Docker image with hindent for ease of use

[evolutics]

Completed checklist

• I've checked the issue tracker for similar issues.
• I am not creating an issue about adding more configuration options for the styling/layout of hindent, because hindent is not about customizing style.

Problem description

Easily running a given version of hindent in an environment with Docker, for example for continuous integration. This should avoid issues such as #563 and #497.

Proposed solution

On each release of hindent (say, on Git tag 7.8.9), add a Docker image mihaimaruseac/hindent:7.8.9 and mihaimaruseac/hindent:latest to Docker Hub.

Alternatives

Statically-linked binary for Linux.