Update docker image and re-add anaconda user

[justinGilmer]

PR Summary:

Previously, the latest version of miniconda3 alpine docker image was incorrectly built as a debian image. This has now been fixed.

However, in between the previous version we have been using, the anaconda user was not added by default. I added the necessary adduser commands to fix this.

PR Checklist

---

• [ ] Includes appropriate unit test(s)
• [ ] Appropriate docstring(s) are added/updated
• [ ] Code is (approximately) PEP8 compliant
• [x] Issue(s) raised/addressed? Closes #453