Make all fields optional

README.md

@@ -5,6 +5,10 @@ Start containers specified in a docker-compose file.
 
 ## Inputs
 
+All inputs are optional, but either credentials for the Docker Hub or
+Amazon Elastic Container Registry (Amazon ECR) are recommended to avoid
+issues with rate limits.
+
 ### `docker-hub-username`
 
 The username used for login to Docker Hub.
@@ -47,7 +51,8 @@ container must be named `db`.
 
 ### `working-directory`
 
-**Optional** Specifies the working directory where the commands are run.
+Specifies the working directory where the commands are run.  Uses `./` by
+default.
 
 
 ## Example usage

action.yml

@@ -2,8 +2,10 @@
 inputs:
   docker-hub-username:
     description: Username for Docker Hub login
+    default: ''
   docker-hub-access-token:
     description: Access token for Docker Hub login
+    default: ''
   aws-access-key-id:
     description: Access key for AWS user with Amazon ECR access
     default: ''
@@ -18,8 +20,10 @@ inputs:
     default: ''
   service-containers:
     description: Service containers to start
+    default: ''
   db-name:
     description: Database name
+    default: ''
   apply-migrations:
     description: Set to true to apply migrations
     required: false