Restore fixture allowing usage for local testing

This reverts commit dc0b60e63f and allows using `--prebuild` CLI flag for pytest when doing local tests.

README.md

@@ -153,21 +153,32 @@ poetry install
 
 ### Testing
 
-To run the tests locally, you first need to build the image locally:
+To run the tests locally, add `--prebuild` to autobuild the image before testing:
 
-```
-docker build -t docker-socket-proxy:local .
+```sh
+poetry run pytest --prebuild
 ```
 
-You can then run them with:
+By default, the image that the tests use (and optionally prebuild) is named
+`docker-socket-proxy:local`. If you prefer, you can build it separately before testing,
+and remove the `--prebuild` flag, to run the tests with that image you built:
 
-```
+```sh
+docker image build -t docker-socket-proxy:local .
 poetry run pytest
 ```
 
-_Note:_ You can use the docker tag you want, but that is the one that is picked by
-default in the tests. If you opt for a different one, set the environment variable
-`DOCKER_IMAGE_NAME` to the value you prefer before running the tests.
+If you want to use a different image, export the `DOCKER_IMAGE_NAME` env variable with
+the name you want:
+
+```sh
+# To build it automatically
+env DOCKER_IMAGE_NAME=my_custom_image poetry run pytest --prebuild
+
+# To prebuild it separately
+docker image build -t my_custom_image .
+env DOCKER_IMAGE_NAME=my_custom_image poetry run pytest
+```
 
 ## Logging
 

tests/conftest.py

@@ -2,13 +2,28 @@ import json
 import os
 from contextlib import contextmanager
 from logging import info
+from pathlib import Path
 
+import pytest
 from plumbum import local
 from plumbum.cmd import docker
 
 DOCKER_IMAGE_NAME = os.environ.get("DOCKER_IMAGE_NAME", "docker-socket-proxy:local")
 
 
+def pytest_addoption(parser):
+    """Allow prebuilding image for local testing."""
+    parser.addoption("--prebuild", action="store_const", const=True)
+
+
+@pytest.fixture(autouse=True, scope="session")
+def prebuild_docker_image(request):
+    """Build local docker image once before starting test suite."""
+    if request.config.getoption("--prebuild"):
+        info(f"Building {DOCKER_IMAGE_NAME}...")
+        docker("build", "-t", DOCKER_IMAGE_NAME, Path(__file__).parent.parent)
+
+
 @contextmanager
 def proxy(**env_vars):
     """A context manager that starts the proxy with the specified env.