Add multiple kubernetes tools and re-organized Dockerfile

[Taegost]

• Added the following kubernetes tools:
  • kubeseal - Manages kubernetes SealedSecrets - https://github.com/bitnami-labs/sealed-secrets for more details
  • helm - Manage resource installs - https://github.com/helm/helm
  • kubelogin - Azure login for kubernetes - https://github.com/Azure/kubelogin
  • kustomize - Another method for managing resource installs - https://github.com/kubernetes-sigs/kustomize
• Re-organized Dockerfile to what I hope will reduce the amount of layer rebuilds with each new version of the image
  • Moved all version pin ARGs from the top of the Dockerfile to their respective tool install section
• Commented out mount lines in the example devcontainer.json so the default behavior is, well, default
• Updated README with the new tool and removed mount lines from the example there as well

[Taegost]

This PR also fixes #3