Harden compose secrets and add required provisioning docs

.env.example

@@ -0,0 +1,9 @@
+# Copy this file to default-environment.env (non-secret defaults) and update values.
+PROJECT_ROOT=/path/to/docker
+DOMAIN=example.com
+TZ=Etc/UTC
+EMAIL=admin@example.com
+
+# Required secret file path used by compose services.
+# Create this file from secrets/.env.secrets.example and keep it out of git.
+SECRETS_ENV_FILE=${PROJECT_ROOT}/secrets/stack-secrets.env