🔊 Add consistent errors for env vars not set (#200)

{{cookiecutter.project_slug}}/README.md

@@ -489,10 +489,10 @@ services:
     deploy:
       placement:
         constraints:
-          - node.labels.${STACK_NAME}.app-db-data == true
+          - node.labels.${STACK_NAME?Variable not set}.app-db-data == true
 ```
 
-note the `${STACK_NAME}`. In the script `./scripts/deploy.sh`, the `docker-compose.yml` would be converted, and saved to a file `docker-stack.yml` containing:
+note the `${STACK_NAME?Variable not set}`. In the script `./scripts/deploy.sh`, the `docker-compose.yml` would be converted, and saved to a file `docker-stack.yml` containing:
 
 ```yaml
 version: '3'
@@ -506,6 +506,8 @@ services:
           - node.labels.{{cookiecutter.docker_swarm_stack_name_main}}.app-db-data == true
 ```
 
+**Note**: The `${STACK_NAME?Variable not set}` means "use the environment variable `STACK_NAME`, but if it is not set, show an error `Variable not set`".
+
 If you add more volumes to your stack, you need to make sure you add the corresponding constraints to the services that use that named volume.
 
 Then you have to create those labels in some nodes in your Docker Swarm mode cluster. You can use `docker-auto-labels` to do it automatically.
@@ -632,10 +634,10 @@ You can do the process by hand based on those same scripts if you wanted. The ge
 ```bash
 # Use the environment variables passed to this script, as TAG and FRONTEND_ENV
 # And re-create those variables as environment variables for the next command
-TAG=${TAG} \
+TAG=${TAG?Variable not set} \
 # Set the environment variable FRONTEND_ENV to the same value passed to this script with
 # a default value of "production" if nothing else was passed
-FRONTEND_ENV=${FRONTEND_ENV-production} \
+FRONTEND_ENV=${FRONTEND_ENV-production?Variable not set} \
 # The actual comand that does the work: docker-compose
 docker-compose \
 # Pass the file that should be used, setting explicitly docker-compose.yml avoids the
@@ -653,7 +655,7 @@ config > docker-stack.yml
 docker-auto-labels docker-stack.yml
 
 # Now this command uses that same file to deploy it
-docker stack deploy -c docker-stack.yml --with-registry-auth "${STACK_NAME}"
+docker stack deploy -c docker-stack.yml --with-registry-auth "${STACK_NAME?Variable not set}"
 ```
 
 ### Continuous Integration / Continuous Delivery