Use test image tag for docker-compose tests

Also adds the ability to overwrite the image in docker-compose.yml,
which allows the CI build to use the same image for all docker tests.
The default is still 'benbusby/whoogle-search' though.

.github/workflows/docker_main.yml

@@ -23,6 +23,6 @@ jobs:
     - name: build and test (docker-compose)
       run: |
         docker rm -f whoogle-search-nocompose
-        docker-compose up --detach
+        WHOOGLE_IMAGE="whoogle-search:test" docker-compose up --detach
         sleep 15
         docker exec whoogle-search curl -f http://localhost:5000/healthz || exit 1

.github/workflows/docker_tests.yml

@@ -21,6 +21,6 @@ jobs:
     - name: build and test (docker-compose)
       run: |
         docker rm -f whoogle-search-nocompose
-        docker-compose up --detach
+        WHOOGLE_IMAGE="whoogle-search:test" docker-compose up --detach
         sleep 15
         docker exec whoogle-search curl -f http://localhost:5000/healthz || exit 1