add DOCKER_USER_AGENT to env example

.env.example

@@ -1,5 +1,6 @@
 APPLICATION_NAME="My Deno Application"
 VERSION=0.1.0
 DEBUG_LEVEL=DEBUG # DEBUG, INFO, WARNING, ERROR, CRITICAL
+DOCKER_USER_AGENT="${APPLICATION_NAME}/${VERSION} (Docker; HealthCheck)"
 
 GREETING="hello world!"