docker vitess/vtctlclient: Install curl
docker vitess/vtctlclient: Install curl

Install curl into the docker image vitess/vtctlclient. That way,
for clients using this container, not only can they interact
with vtctld via vtctlclient, but they can also interact with
vtctld and other Vitess components via their HTTP APIs, and can
also interact with the Kubernetes API server over HTTP.

When tested with Docker Desktop for Mac, Docker version 20.10.2,
using the command

        docker build --compress -f Dockerfile -t "vitess/vtctlclient:latest" .

the original Dockerfile generates an artifact that is 94.4MB
(according to docker image ls), whereas the new Dockerfile
generates a 102MB image, about an 8% increase in the image's
size.

Signed-off-by: Jordan Moldow <2e61a635905d7e2b268877701cf7bcae5fba761e@alum.mit.edu>

tinyspeck/vitess,mahak/vitess,vitessio/vitess,vitessio/vitess,vitessio/vitess,vitessio/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,tinyspeck/vitess,vitessio/vitess,mahak/vitess,tinyspeck/vitess,mahak/vitess,tinyspeck/vitess,tinyspeck/vitess,tinyspeck/vitess,tinyspeck/vitess