Github action supporting build multi-arch images

[timtorChen]

This PR is for github action supporting auto build multi-arch images (issue #65).

The build will be triggered while

1. you push a commit on master branch
2. you push a tag on a commit

As the result, there will be three kind of image tag, the semver tag like 0.2.5, the latest tag refering the newest semver, and the edge tag refering top most commit on master branch.

Note that

Before merging, please add secrets DOCKER_USERNAME and DOCKER_TOKEN to the project secrets, enabling login into docker hub.

[Place1]

Hey, thanks for this PR. This is really great!

If you don't mind i'll merge this and then change it a little bit.

I'd like to change the following:

• rename the edge tag to master so that it's the same name as the gh branch it tracks
• change the tag wildcard to v*.*.* to hopefully catch any mistakes :)
• update the moby/buildkit image to v0.8-beta

[timtorChen]

Glad to help this project, and just a reminder, tag as v*.*.* will break the tagging convention you previously use.