Update tini install snippet

[Firefishy]

• Update tini to v0.18.0 (current latest release)
• Add gpg recv-keys retry loop per Guidance on GPG best practices #4252
• Switch to tini-static instead of dynamic build for automatic compatibility with alpine
• Use tini --version instead of tini --help which is not available in TINI_MINIMAL releases.

Snippet tested on:

• Ubuntu 12.04 to 18.04
• Debian
• Alpine

[tianon]

I'm not opposed to updating this, but I think it needs a bit more work (as I've noted in a few comments below).

Would you like to continue working on this, or would you prefer I take over from here?

[Firefishy]

Thank you for the awesome detailed PR comments. I have updated based on the comments.

• Added notes in snipped to use distro tini if available
• Switched to wget
• Removed loop and used standard || logic (Bonus: works with centos /bin/sh now too)
• Used gpgconf --kill all logic and removed the -f on rm
• set -eux now and switched to using ;

Updated snipped tested on:

• Ubuntu 12.04 to 18.10
• Debian
• Alpine
• Centos 6+

[Firefishy]

@tianon would you mind reviewing when you have a moment.

[yosifkit]

Added my suggestions.

[Firefishy]

@yosifkit Thank you for the review. I have merged and squashed all the recommended changes.