Build Docker images using goreleaser-pro instead of make

[tashian]

Note: We need to wait for a release of GoReleaser Pro before we can merge this.

This PR builds bin/step using the same GoReleaser Pro workflow as in other parts of the CI system:

goreleaser build \
      --id default \
      --single-target \
      --auto-snapshot \
      --clean \
      --output bin/step

It should use the version number from the most recent tag.
The --auto-snapshot flag appends -next to the version number if the git repo is dirty.

The --single-target flag builds for the current GOOS, GOARCH, and GOARM only.

• Test that Docker images build properly across different platforms
• Do a prerelease build, to see the whole thing working properly