Fix Docker image.

[russjones]

Purpose

Trying to start Teleport using DynamoDB as the backend would result in the following error:

ERROR REPORT:
Original Error: *awserr.baseError RequestError: send request failed
caused by: Post https://dynamodb.us-west-2.amazonaws.com/: x509: certificate signed by unknown authority

This was caused by missing CA certificates in the Docker image.

Implementation

• Install ca-certificates in Docker image.
• Re-factor Dockerfile to add comments for each layer.
• Downgrade to LTS Ubuntu release.

[klizhentas]

@russjones while we are on this topic, can you add tsh to the image as well?

#2467