Add healthcheck endpoint, set default ENV in Dockerfile

[robwatkiss]

This pull request adds an optional healthcheck endpoint, useful for environments such as Kubernetes where readiness needs to be probed and monitored.

Four new environment variables are introduced:

• SERVER_HEALTHCHECK_ENABLED - optionally enable a static healthcheck endpoint by setting SERVER_HEALTHCHECK_ENABLED=1
  • useful for deploying to Kubernetes or similar infrastructure where readiness needs to be confirmed and monitored
  • disabled by default
• SERVER_HEALTHCHECK_PATH - optionally override the location of the healthcheck endpoint
  • defaults to /healthcheck
• SERVER_HEALTHCHECK_RESPONSE_CODE - optionally override the status code of the healthcheck endpoint response
  • defaults to 200
• SERVER_HEALTHCHECK_RESPONSE_BODY - optionally override the body of the healthcheck endpoint response
  • defaults to alive

If SERVER_HEALTHCHECK_ENABLED=1 then /etc/nginx/includes/healthcheck.conf is included into default.conf.

In order for location matching against /${SERVER_HEALTHCHECK_PATH} it was necessary to wrap existing redirects in

location / {
    ...
    # Existing redirect directives
    ...
}

Finally this pull request tidies up run.sh by declaring default ENV values at build time in the Dockerfile.

[schmunk42]

@robwatkiss Is this backward-compatible?