backend/docker-compose: add pg dependency, optional persistence

This change adds a dependency to the (nebraska) "server" service on the
"postgres" service in docker-compose.test.yaml.
Motivation of the change is to prevent a race condition where the
"server" container starts faster than "postgres", fails to connect to
the database, and shuts down.

Additionally, an optional persistent storage for postgres is added.
DB contents can be persisted across docker-compose runs to aid manual,
interactive testing. Usage instructions have been added to the YAML
files as comments. The local directory used for persiting postgres data
has been added to .gitignore to prevent accidental commits.

Signed-off-by: Thilo Fromm <thilofromm@microsoft.com>

.gitignore

@@ -1,3 +1,4 @@
 .DS_Store
 *.swp
 *sublime*
+backend/__postgres_testing_volume/

backend/docker-compose.pg-persist.yaml

@@ -0,0 +1,8 @@
+# Persistent postgres storage drop-in for interactive testing.
+# This drop-in will persist postgres data in $(pwd)/__postgres_testing_volume
+#  across restarts of docker-compose.
+
+services:
+  postgres:
+    volumes:
+      - ./__postgres_testing_volume:/var/lib/postgresql/data

backend/docker-compose.test.yaml

@@ -1,3 +1,15 @@
+# Docker-compose file for interactive and for automated testing.
+#
+# Build docker image from local sources:
+#   docker-compose --file docker-compose.test.yaml build
+#
+# Run:
+#   docker-compose --file docker-compose.test.yaml run
+# Run with persistent storage:
+#   docker-compose --file docker-compose.test.yaml --file docker-compose.test-db-persist.yaml up
+#
+# See docker-compose.pg-persist.yaml for more informatino on persistent storage.
+
 version: "3.9"
 
 services:
@@ -21,6 +33,8 @@ services:
       context: ../
       dockerfile: Dockerfile
       network: host
+    depends_on:
+      - postgres
     ports:
       - "8002:8000"
     depends_on: