Run as a non-root user in Docker

**Describe the solution you'd like**

It is better not to run as root in Docker containers where it is not needed, for security reasons. Currently, the Docker image runs as root by default:

```
docker run --rm --entrypoint whoami zricethezav/gitleaks:v8.30.1@sha256:c00b6bd0aeb3071cbcb79009cb16a60dd9e0a7c60e2be9ab65d25e6bc8abbb7f
root
```

cc @zricethezav

Start by locating the Docker image build definition for zricethezav/gitleaks:v8.30.1 and check how its default runtime user is set. Verify the image with the reported docker run --entrypoint whoami command, confirm it no longer reports root, and ensure the CLI still runs as expected.

Written by the indexing model from the issue text.