dev/sg: clarify behaviour of -t='', optimize getUpdatedImage

Created by: bobheadxi

@kalanchan and I were both confused by what "latest deveopment build" means - to clarify, we formalize the tag format and:

• rename various generic references to tag to indicate what they really refer to (main branch tag)
• update the help text
• remove unnecessary calls to Docker's list tags API in getUpdatedImage if -t is set

Test plan

Only renames + reading of fetchAllTags indicates no behavioural changes with the move