local docker: copy examples/common into /vt/common to match MoveTables user guide

[mackenziestarr]

Description

The current MoveTables user guide https://vitess.io/docs/user-guides/migration/move-tables/ doesn't work with the local dockerfile setup as the guide expects SQL files to be available in the relative path ../common. Because that directory isn't copied into the top-level /vt directory they aren't available for the entrypoint working directory. This PR copies examples/common into /vt/common to make the docker environment match the user guide expectations.

I'd also note that I tested this on the v8.0.0 tag, as make docker_local was broken for me the latest master:HEAD ea65a1c with Docker version 1.13.1 (admittedly an older version):

$ make docker_local
Building vitess/local
Sending build context to Docker daemon 235.2 MB
Step 1/22 : ARG bootstrap_version=0
Please provide a source image with `from` prior to commit
make: *** [docker_local] Error 1

could be related to #7182, happy to open a separate issue for it

Related Issue(s)

Checklist

• Should this PR be backported?
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving
• VReplication
• Cluster Management
• Build

[dkhenry]

LGTM