feat: Add non-root user to Dockerfile for security

[JasonShigemura]

Configure Docker image to run as 'appuser' instead of root.

Description

This PR addresses a critical security concern by configuring the Docker container to run the application as a dedicated, unprivileged non-root user ('appuser') instead of the default 'root' user.

Running containers as root is against security best practices as it increases the potential impact if the application process is compromised. Switching to a non-root user limits the process's privileges within the container.

This change aligns with Docker security recommendations and hardens the deployment environment.

Changes Made

• Added instructions in the release stage of the Dockerfile to:
  • Create a new group (appgroup) and user (appuser).
  • Change ownership of the /app directory to appuser:appgroup.
• Added the USER appuser instruction before the ENTRYPOINT to switch to the non-root user.

Related Issues/PRs

• Complements the work in potential open PR Add CI that tests docker build and run #18 (Add CI that tests docker build and run) by ensuring the tested Docker image follows security best practices.

Checklist

• Dockerfile modified to add and use a non-root user.
• Docker image built and tested locally (docker build .)