Multi-stage Docker image build

[sajith]

Resolves #176. The final image should be a few hundreds of MB smaller than what we currently have.

[coveralls]

Pull Request Test Coverage Report for Build 9424774367

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 52.132%

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Totals
Change from base Build 9319526474:	0.0%
Covered Lines:	738
Relevant Lines:	1446

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💛 - Coveralls

[sajith]

From local testing, on main branch:

$ docker inspect -f "{{ .Size }}" sdx-controller | numfmt --to=si
1.6G

On this PR branch:

$ docker inspect -f "{{ .Size }}" sdx-controller | numfmt --to=si
695M