fix checksum check

[alexander-dammeier]

the builder image is currently not used. This is no problem at the moment as the copied file directly comes from the repository. We also waste 30MB of space because of the separate extraction of doguctl.

We should do the following:

• in build stage
  • download doguctl from github
  • do checksum
  • extract it
• in main image:
  • just copy the extracted doguctl cli from build stage
• delete doguctl from the repository