Enhancement: Refactor to use one set of Docker instruction name constants when processing Docker instruction/command info

[kcq]

We define constants for the Dockerfile instruction names in multiple places. Example:

https://github.com/mintoolkit/mint/blob/master/pkg/docker/instruction/instruction.go#L9 (e.g., Cmd = "cmd")
https://github.com/mintoolkit/mint/blob/master/pkg/docker/dockerfile/dockerfile.go#L19 (e.g., instTypeCmd = "CMD")
https://github.com/mintoolkit/mint/blob/master/pkg/docker/dockerfile/reverse/reverse.go#L108 (e.g., instTypeCmd = "CMD")

We need to have/use one set of Dockerfile instruction constants and we need to use this one set of names in the related code.