Build docker images on all builds and publish on tag

[MikeGoldsmith]

Which problem is this PR solving?

Forked builds will fail the publish_docker step because it tries to login to docker, but the username and password are in CircleCI secrets that are not shared on forked builds.

• Resolves testing docker image build+publish fails from forks #325

Short description of the changes

• rename jobs to be clearer
• update build-docker.sh to only ever build & tag docker images and publish locally (ko.local)
• store built image in workspace and as an artefact
• for tagged builds, publish stored image with tags

Also includes some maintenance tasks for the circle config:

• remove custom executor as only one job used it
• fix whitespace issues

Known issue:

• googkleko should be cached between workflows, but $GOPATH is not expanded in the save_cache step, which means we re-install on every run

[vreynolds]

as a future optimization: we can cache dockerize and ko if we download the executables and cache those in a temp directory ala buildevents: https://github.com/honeycombio/buildevents-orb/blob/main/orb.yml

[MikeGoldsmith]

as a future optimization: we can cache dockerize and ko if we download the executables and cache those in a temp directory ala buildevents: https://github.com/honeycombio/buildevents-orb/blob/main/orb.yml

Thanks for the suggestion - I've created #332 to track caching those 👍🏻