feat(image-builder): Add field to allow api server env vars to be propagated to kaniko jobs

[deadlycoconuts]

Description

This PR introduces a new mechanism for environment variables from the Merlin API server to be propagated to the build environment of the Kaniko build jobs that it spins up, reducing the need for redundant repetition of configuration, especially if these environment variables are common to both the Merlin API server and the build environment of the Kaniko build jobs.

Since these variables are passed to the Kaniko build jobs as build arguments (as opposed to environment variables of the container where the image gets built; see GoogleContainerTools/kaniko#2824 for more details), these variables get passed as additional arguments in the Kaniko build job.

This PR additionally introduces a cluster connectivity check (done by a simple list pods operation) for the image builder to ensure that the cluster controller has been successfully initialised at start up of the Merlin API server, instead of allowing errors to only be thrown when an image building job has been triggered.

Modifications

• api/cmd/api/setup.go - Addition of a connectivity check during initialisation of the image builder
• api/pkg/imagebuilder/imagebuilder.go - Addition of a step to add Merlin API server environment variables as Kaniko build args

Tests

Checklist

• Added PR label
• Added unit test, integration, and/or e2e tests
• Tested locally
• Updated documentation
• Update Swagger spec if the PR introduce API changes
• Regenerated Golang and Python client if the PR introduces API changes

Release Notes

NONE

[bthari]

Just one tiny comment, but the rest LGTM, so feel free to merge it once it's addressed. Thank you for the changes and improvement! 🚀

[deadlycoconuts]

Thanks for the quick review and for spotting the issue with the comments! Merging this now! 🚀