Add mysqlctl docker image

[keweishang]

Description

The Vitess Connector is using the vitess/base docker image as the base image to copy useful binaries, such as mysqlctl, vtctl, etc. The vitess/base docker image is huge (6.56GB). In order to avoid downloading this huge image, this PR creates the mysqlctl docker image, so that users can copy mysqlctl from this much smaller docker image.

The new docker image vitess/mysqlctl should be pushed to DockerHub.

Related Slack discussion: https://vitess.slack.com/archives/C0PQY0PTK/p1612339740175400

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving
• VReplication
• Cluster Management
• Build/CI
• VTAdmin

[keweishang]

@deepthi Could you have a look at this PR? Thanks

[keweishang]

I'm not sure if the directory /vt/config is needed here? I'm copying it here because the mysqlctld docker image has it too.

Ideally, it'd be great if this docker image is more useful than just serving my purpose, which is to copy the /vt/bin/mysqlctl binary from this image (e.g. see example here).

[deepthi]

/vt/config contains mysql .cnf files so it is probably required.

[deepthi]

I'm going to approve this as-is, though in the long-term we should stop using the k8s base image altogether and unify / reduce the number of docker images we produce.
cc @derekperkins

[keweishang]

Shall we merge the PR? @derekperkins @deepthi

[deepthi]

Shall we merge the PR? @derekperkins @deepthi

I'm about to. I will build and push a docker image later today.

[deepthi]

I've pushed a vitess/mysqlctl:latest image to dockerhub. For now, this is a one-time build. We'll do a tagged version along with the next release.

[keweishang]

Thank you very much @deepthi! Having the vitess/mysqlctl:latest image available on dockerhub should work for us. I'll test it this or next week.