feat: Create multi-arch images

[tgtajuckel]

This is an initial straw man implementation. I'm expecting to clean up source, testing, and commit messages before creating a final PR for merging. The goal for this draft PR of early work is merely to solicit feedback. Some bits remain awkward. The way the plugin is deciding whether to execute Build or Manifest commands is a total kludge, for instance.

Specific questions I'd appreciate feedback on:

• Is the embedded manifest: configuration acceptable? Does anyone have suggestions for improvement?
• Does anyone have experience providing or consuming --os-features flags in a list manifest? The docs list it as an array of strings, but I'm not sure if a simple comma-separated list is what is expected. Perhaps I should provide multiple --os-features arguments? I've just honestly not encountered it.
• Would it be better to move move this work to a separate vela-docker-manifest plugin? I'm hesitant to do so, as the entire repository infrastructure is shared, but the current build/manifest dichotomy feels very awkward.

This is an attempt to resolve go-vela/community#482

[kneal]

Would it be better to move move this work to a separate vela-docker-manifest plugin? I'm hesitant to do so, as the entire repository infrastructure is shared, but the current build/manifest dichotomy feels very awkward.

I think it makes sense to do this work here to keep the plugin consolidated. We have a lot of plugins that use a multi command structure for binaries.

[codecov]

Codecov Report

Merging #49 (251bf3c) into master (d9656b7) will decrease coverage by 8.80%.
The diff coverage is 28.72%.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
- Coverage   62.89%   54.08%   -8.81%     
==========================================
  Files           7        8       +1     
  Lines         671      832     +161     
==========================================
+ Hits          422      450      +28     
- Misses        241      367     +126     
- Partials        8       15       +7     

Impacted Files	Coverage Δ
cmd/vela-docker/main.go	0.00% <0.00%> (ø)
cmd/vela-docker/plugin.go	17.44% <13.63%> (-0.31%)	⬇️
cmd/vela-docker/manifest.go	32.00% <32.00%> (ø)
cmd/vela-docker/build.go	86.28% <100.00%> (-9.63%)	⬇️