Added multiarch Teleport Lab support

[fheinecke]

This is the third and final part of a series of feature PRs updating drone/dronegen to support multiarch builds.

The primary purpose of this PR is to add multiarch Teleport Lab container images. This includes amd64, arm, and arm64 images under the $MAJOR_VERSION tag on each release.

This includes three sets of minor changes outside of Teleport Lab specific dronegen changes:

• Refactored the Teleport Lab dockerfile to be based on the locally built Teleport images. This does not require any Makefile changes as we have no Makefile targets for Teleport Lab.
• Refactored a couple of container image functions:
  • downloadTeleportDockerfileStep was renamed to downloadDockerfileStep and the produced step name was simplified due to Drone step name length limitations
  • If Product ArchSetupSteps contain no commands, then the empty step is not added to the pipeline but treated as a dependency step
• Removed these pipelines in their entirety as their functionality has been completely replaced by the new container image pipelines:
  • teleport-docker-cron
  • teleport-docker-cron-ecr

Per usual there is a large number of .drone.yml changes. Here's a breakdown of what changed:

• 226 lines of old pipelines were removed
• 16 lines were changed downloadTeleportDockerfileStep refactoring
• 204 lines were added to the pipeline that builds new images
• 396 lines were added to the pipeline that pulls/pushes images to production registries upon promotion
• 619 lines were added to the pipeline that rebuilds/updates images across all repos, per each of the last three major versions, totaling 1857 lines
• The remaining lines appear to be due to git diff matching up the two file versions in different places. See here for an example.

Related PRs:

• Added multiarch build support for Teleport #17597
• Added multiarch build support for teleport-operator #16688
• Added multiarch Teleport container images #15514
• Request: ARM container builds #3384

Test runs prior to merging PR:

• Tag logic with empty container repo (v1.2.3-fred.12): https://drone.platform.teleport.sh/gravitational/teleport/17140/27/1
• Tag logic again with the same tag, via retag (v1.2.3-fred.12): https://drone.platform.teleport.sh/gravitational/teleport/17143/27/1
• Promote logic with exit due to prerelease (v1.2.3-fred.12): https://drone.platform.teleport.sh/gravitational/teleport/17150
• Promote logic with empty container repos (v1.2.3-fred.12): https://drone.platform.teleport.sh/gravitational/teleport/17156
• Promote logic with the same tag, via restart (v1.2.3-fred.12): https://drone.platform.teleport.sh/gravitational/teleport/17157
• Cron logic test (v1.2.4-fred.11): pending

[fheinecke]

This PR is suspended pending a resolution to the drone.yml file size limitation.