Add ability to pull images by digest

[Boojapho]

Currently only tags can be used to pull images. In my use case, I am bundling images for an air-gapped deployment into an OCI image using Porter. This requires the images to be referenced by digest rather than tag. Other use cases include wanting to deploy immutable containers (tags can be changed, SHA digests cannot).

The Helm charts should be updated to support a digest. In other charts I have seen, the digest will override the tag if present. In addition, the digest includes the full digest string (e.g. "sha256:12345abcde...") as the value.