Remove the requirement on docker-compose for running tests

Besides simplifying the setup it also has the nice side-effect of
being able to override the docker image on the command line:
$ make test IMAGE=foobar

.github/CONTRIBUTING.md

@@ -207,7 +207,7 @@ these principles when making changes.
     ```
 
 4. Verify you can run the test harness. _(This will require dependencies:
-   `make`, `docker`, and `docker-compose`)_.
+   `make` and `docker`)_.
 
     ```text
     $ make test

Makefile

@@ -93,12 +93,8 @@ test:
 		cd /yadm && \
 		py.test -v $(testargs); \
 	else \
-		if command -v "docker-compose" > /dev/null 2>&1; then \
-			docker-compose run --rm testbed make test testargs="$(testargs)"; \
-		else \
-			echo "Sorry, this make test requires docker-compose to be installed."; \
-			false; \
-		fi \
+		$(MAKE) -s require-docker && \
+		docker run --rm -it -v "$(CURDIR):/yadm:ro" $(IMAGE) make test testargs="$(testargs)"; \
 	fi
 
 .PHONY: testhost

docker-compose.yml

@@ -1,7 +0,0 @@
----
-version: '3'
-services:
-  testbed:
-    volumes:
-      - .:/yadm:ro
-    image: yadm/testbed:2020-12-21