Adding imageTag helper

[maxirus]

This PR will serve as the basis to fix #24. Unfortunately, the Frigate Docker image is not built with multi-arch and instead is built separately with a unique tag to denote each CPU architecture. Because of this, the tag or more importantly the image pull, cannot be done in a standard fashion.

This means:

• user must know the specific cpu architecture for the node the container will land on
• User needs to lookup the proper tag to use
• Will not allow for K8s deployments in a mixed architecture environment
• Harder to manage updates
• More config options a user must input to get started

With a multi-arch Docker image, the daemon automatically pulls the proper image for the CPU architecture and thus having a single tag. I've created a new issue here

[onedr0p]

Looks good, I would just bump the minor version not the patch version of the chart :)