fix: do not delete devcontainers when running tests in a docker container

.devcontainer/Dockerfile

@@ -15,4 +15,5 @@ RUN make ci-dependencies
 
 COPY . .
 
-ENV DOKKU_HOSTNAME=dokku.me
+ENV DOKKU_HOSTNAME=dokku.me
+LABEL org.label-schema.schema-version=1.0 org.label-schema.vendor=dokku com.dokku.devcontainer=true

tests/unit/test_helper.bash

@@ -43,7 +43,7 @@ cleanup_apps() {
 cleanup_containers() {
   containers=$(docker container ls --quiet)
   if [[ -n "$containers" ]]; then
-    docker container ls --quiet | xargs -n1 docker container rm -f || true
+    docker inspect -f '{{ if ne "true" (index .Config.Labels "com.dokku.devcontainer") }}{{.ID}} {{ end }}' $(docker ps -q) | xargs --no-run-if-empty -n1 docker container rm -f || true
   fi
 }