Minor Dockerfile Improvements

[theinfosecguy]

Description

• Added the --rm flag to apk add to remove the package cache after installation.
• Set the CGO_ENABLED environment variable to 0 to disable CGO and produce a static binary.
• Created a new user and group named chainbench in the final image.
• Used the USER directive to switch to the chainbench user before running the application.

Related issues

• No issues

Related PRs

• No issues

Remove this section if you don't have related PRs.

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the readme with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}