feat(client): Add custom User-Agent in Docker client as tc-python/<version> (testcontainers#507)

Set User-Agent in format `tc-python/<version>`, `version` value is
coming from `pyproject.toml`.

The `User-Agent` header will allow to identify Testcontainers language
implementation and the specific version. Also, track the usage of the
library.

Author: eddumelendez

core/testcontainers/core/docker_client.py

@@ -11,6 +11,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 import functools as ft
+import importlib.metadata
 import ipaddress
 import os
 import urllib
@@ -44,6 +45,7 @@ def __init__(self, **kwargs) -> None:
         else:
             self.client = docker.from_env(**kwargs)
         self.client.api.headers["x-tc-sid"] = SESSION_ID
+        self.client.api.headers["User-Agent"] = "tc-python/" + importlib.metadata.version("testcontainers")
 
     @ft.wraps(ContainerCollection.run)
     def run(