Speed up plugin downloads

[miceg]

This PR is based on #658, that'll need to be merged first to get a clean diff.

Fixes: #642

This rearchitects the image's plugin and component downloads:

• Clean builds in my organisation's CI infrastructure went from 8 minutes to 4 minutes, and rebuilds take only 2 minutes.
• Clean builds in GitHub CI look to have dropped from around 15-20 minutes (and intermittently failing) to 7 minutes.

Detailed change list:

• Use curl rather than wget, because wget1 doesn't support HTTP/2 and friends (which are faster).

• Download plugins using a separate Docker image, using a single curl process to benefit from connection re-use:

  • This runs in parallel with the main image – clean builds can download the plugins while installing dependencies from apt.
  • This benefits from Docker image caching on rebuilds – if the plugins list doesn't change, you don't have to download another 500MB of stuff.
  • This now propagates errors, rather than silently failing.

• Remove validate_url (which called wget twice), and just call curl directly.

• Make download_extension only try to download each extension once using curl, rather than twice using curl and wget.

• Simplify package_geoserver so it doesn't repeatedly repeat itself, and doesn't fetch each URL twice. Also makes it exit 1 on errors.

• Clean up some strange shell syntax.

There are still some error handling weaknesses in the main image – the proper solution to that would be to use set -e. I'm hesitant to turn that on without further testing, because there may be assumptions in the existing script.

There are still some component downloads in the main image build which could be moved into the plugin downloader (such as GeoServer itself and libjpeg-turbo), but this is still a huge improvement.

[NyakudyaA]

@miceg Are you able to complete this soon? I need to publish the latest bug fix .1 version. If not I will push the new version and we can update this when you get to it

[miceg]

Thanks for finishing that off – I've been stuck in some other work. The changes you made in the final version look good to me. :)