disable docker socket support until we have security in place

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

environment/pipeline.go

@@ -468,11 +468,6 @@ func (p *Pipeline) mount(ctx context.Context, dest string, mnt *compiler.Value)
 				llb.Scratch(),
 				llb.Tmpfs(),
 			), nil
-		case "docker.sock":
-			return llb.AddSSHSocket(
-				llb.SSHID(solver.DockerSocketID),
-				llb.SSHSocketTarget(dest),
-			), nil
 		default:
 			return nil, fmt.Errorf("invalid mount source: %q", s)
 		}