Various improvements mostly related to Docker usage

[ceejatec]

Main program:

• Handle SIGINT/SITGERM better (explicitly terminate openconnect).
  Without this, stopping the Docker container wouldn't allow
  openconnect to restore /etc/resolv.conf.
• Allow one-time code to be specified in conf file/environment

Docker wrapper scripts:

• Use environment to pass one-time code (old approach of overriding
  the secret value in conf file didn't work)
• Identify whether code is needed more generically (if ANY totp.xxxx
  secrets are specified, skip the code)
• Skip commented lines in conf file when looking for secrets, username,
  etc.
• Output appropriate Docker logfiles when stopping and starting
• Add stop-docker.sh script to cleanly bring down VPN, with logs

Docker image:

• Use python 3, based on python/debian-slim base image
• Multi-stage build to keep openconnect build tools out of final image
• Only gp-okta.conf needs to be mounted into image now
• Update URL for retrieving vpnc-script (old git repository has
  gone away)
• Disable buffering for cleaner output

[ceejatec]

@arthepsy do you have any questions or comments about this PR?