Add docker security scan

[darrenvechain]

Description

This step adds a docker security scan after building the image.

In order to make this work, we needed to load the image locally (push does not store the image locally), and this requires that we only build on 1 platform. See issue: https://github.com/docker/buildx/issues/59

Type of change

• [x] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

On this GHA: https://github.com/vechain/thor/actions/runs/8725735636

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] New and existing E2E tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [x] I have not added any vulnerable dependencies to my code

[darrenvechain]

@libotony should we set a low threshold?