Push Docker images as part of the release workflow

[johnmaguire]

Closes #25.

Description

This PR adds a Github Actions workflow to build and push Docker images whenever a new release is tagged. The image is pushed with a version tag, as well as with the latest tag. This allows users to choose between manual updates or automatic updates by pinning to the latest tag.

With this image, you can provide Nebula connectivity to your Docker host while running Nebula inside a container:

docker run \
    --name nebula \
    --network host \
    --cap-add NET_ADMIN \
    --volume ./config:/etc/nebula \
    --rm \
    nebulaoss/nebula

In the example above, you must create a config directory containing a valid config.yml. Here is the same configuration, but with docker-compose:

version: '2'
services:
  nebula:
    image: nebulaoss/nebula
    network_mode: host
    cap_add:
      - NET_ADMIN
    volumes:
      - ./config:/etc/nebula
    restart: unless-stopped

This image also allows overriding the args passed to Nebula:

❯ docker run nebulaoss/nebula -version
Version: 0.0.11

This can be used to allow using a config directory instead of a config file, for example.

Github Actions Details

If DOCKERHUB_USERNAME and DOCKERHUB_TOKEN are not set, every step in the build-docker job of the release workflow is skipped. This avoids a "failed" status in forks that want to create repo-local releases without pushing to Docker. The tradeoff is that it could indicate a false success when the required variables are not configured.

Variables

Required:

• DOCKERHUB_USERNAME (variable)
• DOCKERHUB_TOKEN (secret) - must be read/write

Optional:

• DOCKER_REPO_NAME (variable) - defaults to "nebulaoss/nebula"
• DOCKER_REPO_TAG (variable) - defaults to "latest" (e.g. can be overridden to "unstable")

Todo

• Configure Docker secrets in this repo
• Update nebulaoss/nightly with its own Docker secrets and repo

[johnmaguire]

I just noticed that this branch had old code, and was not up-to-date with my personal repo's branch. I just pushed an update.

[johnmaguire]

@wadey The changes look great. I really like the distroless approach! Thanks for all the help here!