Switch to staged docker build

[mumblepins]

Switches to a staged Dockerfile in order to decrease build time, improve build caching, and decrease image size from ~240 MB to ~130 MB

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

The PR fulfills these requirements:

• All tests are passing?
• New/updated tests are included?
• If any static assets have been updated, has ui/bindata.go been regenerated?
• Are there doc blocks for functions that I updated/created?

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information: