elasticsearch DNS round-robin example doesn't work on arm

[BretFisher]

elasticsearch:2 is not available on arm, and later versions require complex setup steps that we don't want when learning a simple DNS feature, so let's replace that container with httpenv, which has the same effect in learning without the complexity of elasticsearch.

• [ ] Find all uses of elasticsearch in the course and replace with httpenv
• [x] Add a workaround text lecture for now on elasticsearch, centos, and alpine apk (workaround lecture added)
• [ ] Make a readme for the assignment ./docker-assignment-2
• [ ] Replace Assignment DNS Round Robin Test
• [ ] relace elasticsearch:2 example in Lecture "Healthchecks in Dockerfiles"

Original Q&A