Add the assignment to implement a health check

[refs #114c1c04fcd7]

Author: jfahrer

swarm/assignment6.md

@@ -0,0 +1,14 @@
+## Implement health checks
+* Use an image for an application you are familiar with
+  * nginx
+  * ...
+* Implement a health check
+  * curl
+  * wget
+  * ...
+* You might have to extend the image to install the tools you need
+* Setup the check in a Compose file
+  * Verify it works
+* Use the HEALTHCHECK instruction
+  * Verify it works
+* Disable the health check in the Compose file