Update Varnish 6.0 (LTS) and 6.2 versions

[samford]

This updates to the 6.0.4 and 6.2.1 versions of Varnish, which address the VSV00003 DoS attack vector (CVE-2019-15892).

More info here: https://varnish-cache.org/security/VSV00003.html

[pborreli]

lgtm

[samford]

I've updated this PR to the 6.0.5 and 6.2.2 versions of Varnish, which address the VSV00004 workspace information leak. More info here: https://varnish-cache.org/security/VSV00004.html

I've switched to the official Docker image for Varnish but I feel like there's still some value in merging this to update the versions, so long as this remains available on Docker Hub.

[samford]

In the process of updating the official Varnish Docker images (a different repo with the same name) and doing some cleanup after the merge, I accidentally deleted my remote branch for this repo which contained these updates. I've restored the branch, so I'm reopening this.

[samford]

I updated this PR to use versions 6.0.6 (LTS) and 6.2.3 to address VSV00005. I imagine this repo isn't being maintained anymore but I figured I may as well update this anyway.

[soyuka]

ping @teohhanhui ?

[samford]

Version 6.0.7 (LTS) has been released and the download URL now contains a unique string, so this repository's update.sh file doesn't generate the correct URLs anymore. update.sh creates https://varnish-cache.org/_downloads/varnish-6.0.7.tgz (404), whereas the URL from the first-party website that works is https://varnish-cache.org/_downloads/41841608341add28256b374dc367af04/varnish-6.0.7.tgz.

Seeing as this repository hasn't been updated to include the 6.3.x, 6.4.x, or 6.5.x releases and this PR for important security fixes to the 6.0.x and 6.2 series has been open and unmerged for over a year, I'm going to go ahead and close this. Bringing this repository up to date would require more work than what's in this PR and users are better off simply using the official varnish images at this point.