ci: Set Compose project name

Avoids failures while determining container IPs
when the project isn't checked out in a directory
named "docker-elk".

Fixes #839

.github/workflows/ci.yml

@@ -14,6 +14,9 @@ jobs:
     # https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners
     runs-on: ubuntu-22.04
 
+    env:
+      COMPOSE_PROJECT_NAME: docker-elk
+
     steps:
       - uses: actions/checkout@v3