don't include the .git directory in the context send to docker build

without this `DNS_TEST_SUBJECT="hickory $URL" cargo t` was re-building
hickory from scratch even when its source had not change

not including the `.git` directory in the hickory repository lets Docker
see that the source code has not changed

packages/dns-test/src/container.rs

@@ -57,6 +57,9 @@ impl Container {
                 exec_or_panic(&mut cp_r, false);
             }
 
+            fs::write(docker_build_dir.join(".dockerignore"), "src/.git")
+                .expect("could not create .dockerignore file");
+
             exec_or_panic(&mut command, verbose_docker_build());
         });