Add security scan command

[tkilias]

Background

• We need a way to get informed about packages with vulnerabilities
• This should be possible for every flavor
• However, depending on the flavor we might need different tools
• We want to run the security scan mainly for the release step

Acceptance Criteria

• We can run security scan for the release step
• The flavor defines what is running security scan, because this can be different depending on language or package manager

Design idea

• Flavor defines a separate build step which install the scan tools and runs the scan
• The scan build step is based on the release step image (FROM {{ release_image_name }})
• Exaslct implements a new command for the scan of the release image
• parameter to ignore missing security scan build step to allow gradual migration