Enable changing the image tag used with Docker Compose

Add the ability to change the image tag used for the Lambda image in
the Docker Compose configuration. This is done using the LAMBDA_TAG
environment variable with a fallback to the default value of latest.

Author: mcdonnnj

docker-compose.yml

@@ -4,7 +4,9 @@ version: '3.2'
 services:
   build_deployment_package:
     build: .
-    image: "cisagov/example_lambda"
+    # This uses the value of the LAMBDA_TAG environment variable from
+    # the invoking environment but falls back to a default value.
+    image: cisagov/example_lambda:${LAMBDA_TAG:-latest}
     entrypoint: /opt/build_artifact.sh
     environment:
       # This uses the value of the BUILD_FILE_NAME environment variable
@@ -15,6 +17,8 @@ services:
       - .:/var/task/output
   run_lambda_locally:
     build: .
-    image: "cisagov/example_lambda"
+    # This uses the value of the LAMBDA_TAG environment variable from
+    # the invoking environment but falls back to a default value.
+    image: cisagov/example_lambda:${LAMBDA_TAG:-latest}
     ports:
       - "9000:8080"