🔧(backend) add Docker network for shared Keycloak OIDC authentication

Define Docker network enabling external service providers to share Keycloak
instance with local development stack, supporting OIDC authentication flow
where services obtain tokens from shared Keycloak then pass to Meet
for introspection and validation.

Prepares Meet infrastructure for multi-service authentication architecture
though external service provider Docker Compose integration changes remain
in separate repository.

docker/resource-server/compose.yml

@@ -0,0 +1,23 @@
+version: '3'
+
+# You can add any necessary service here that will join the same docker network
+# sharing keycloak. Services added to the 'meet_resource-server' network will be
+# able to communicate with keycloak and the backend on that network.
+services:
+  # busybox service is only used for testing purposes. It provides curl to test
+  # connectivity to the backend and keycloak services. Replace this with your
+  # relevant application services that need to communicate with keycloak.
+  busybox:
+    image: alpine:latest
+    privileged: true
+    command: sh -c "apk add --no-cache curl && sleep infinity"
+    stdin_open: true
+    tty: true
+    networks:
+      - default
+      - meet_resource-server
+
+networks:
+  default: {}
+  meet_resource-server:
+    external: true