Docker - allow BUILD_NUMBER variable override

[guidoiaquinti]

Description

Expose BUILD_NUMBER at Docker build time.

Why

We currently let to override BUILD_NUMBER at build time in a non-Docker environment, I think we should let to do the same in Docker. This will let user to build Vitess, via Docker, using something like:

docker build \
    --no-cache \
    --build-arg BUILD_NUMBER="$BUILD_NUMBER" \
    -t "${IMAGE_TAG}" \
    -f "${GIT_CHECKOUT}/docker/base/Dockerfile.${FLAVOR}" .

[derekperkins]

LGTM

[dkhenry]

👍