Actually target v2021dev3 everywhere

Some changes are merely cosmetic, but two are actually important:

• point to the v2021dev3 image builder Docker image for the GitLab CI image building pipeline
• use v2021dev3 in the README.md examples which are actually tested by the https://qa.apertis.org/sdk-debos-image-building.html testcase

@ritesh, can you poke the branching scripts to be more comprehensive? :)