I tried downgrading libc6 and restarting squid (heck, even rebooting the container), but it still happened. Scott IIRC also tried that.

Still, it's a heck of a coincidence.

Squid in debug mode shows it's getting 10 IPv4 and 10 IPv6 addresses back for the archive, and trying each one in turn. But once it crosses over to the ipv4 ones the log gets a bit confusing for me:
2016/02/20 22:18:32.559| ipcache.cc(497) ipcacheParse: ipcacheParse: 20 answers for 'security.ubuntu.com'
2016/02/20 22:18:32.559| ipcache.cc(566) ipcacheParse: ipcacheParse: security.ubuntu.com #0 [2001:67c:1360:8001::17]
2016/02/20 22:18:32.559| ipcache.cc(566) ipcacheParse: ipcacheParse: security.ubuntu.com #1 [2001:67c:1562::15]
2016/02/20 22:18:32.559| ipcache.cc(566) ipcacheParse: ipcacheParse: security.ubuntu.com #2 [2001:67c:1360:8c01::19]
2016/02/20 22:18:32.559| ipcache.cc(566) ipcacheParse: ipcacheParse: security.ubuntu.com #3 [2001:67c:1360:8c01::18]
2016/02/20 22:18:32.559| ipcache.cc(566) ipcacheParse: ipcacheParse: security.ubuntu.com #4 [2001:67c:1562::14]
2016/02/20 22:18:32.559| ipcache.cc(566) ipcacheParse: ipcacheParse: security.ubuntu.com #5 [2001:67c:1562::17]
2016/02/20 22:18:32.559| ipcache.cc(566) ipcacheParse: ipcacheParse: security.ubuntu.com #6 [2001:67c:1560:8001::13]
2016/02/20 22:18:32.559| ipcache.cc(566) ipcacheParse: ipcacheParse: security.ubuntu.com #7 [2001:67c:1562::13]
2016/02/20 22:18:32.559| ipcache.cc(566) ipcacheParse: ipcacheParse: security.ubuntu.com #8 [2001:67c:1562::16]
2016/02/20 22:18:32.559| ipcache.cc(566) ipcacheParse: ipcacheParse: security.ubuntu.com #9 [2001:67c:1560:8001::11]
2016/02/20 22:18:32.559| ipcache.cc(555) ipcacheParse: ipcacheParse: security.ubuntu.com #10 91.189.91.13
2016/02/20 22:18:32.559| ipcache.cc(555) ipcacheParse: ipcacheParse: security.ubuntu.com #11 91.189.88.153
2016/02/20 22:18:32.559| ipcache.cc(555) ipcacheParse: ipcacheParse: security.ubuntu.com #12 91.189.91.15
2016/02/20 22:18:32.559| ipcache.cc(555) ipcacheParse: ipcacheParse: security.ubuntu.com #13 91.189.91.23
2016/02/20 22:18:32.559| ipcache.cc(555) ipcacheParse: ipcacheParse: security.ubuntu.com #14 91.189.88.152
2016/02/20 22:18:32.559| ipcache.cc(555) ipcacheParse: ipcacheParse: security.ubuntu.com #15 91.189.88.149
2016/02/20 22:18:32.559| ipcache.cc(555) ipcacheParse: ipcacheParse: security.ubuntu.com #16 91.189.92.201
2016/02/20 22:18:32.559| ipcache.cc(555) ipcacheParse: ipcacheParse: security.ubuntu.com #17 91.189.91.24
2016/02/20 22:18:32.559| ipcache.cc(555) ipcacheParse: ipcacheParse: security.ubuntu.com #18 91.189.92.200
2016/02/20 22:18:32.559| ipcache.cc(555) ipcacheParse: ipcacheParse: security.ubuntu.com #19 91.189.91.14

Marks :17 as bad:
2016/02/20 22:18:32.560| ipcache.cc(1075) ipcacheMarkBadAddr: ipcacheMarkBadAddr: security.ubuntu.com [2001:67c:1360:8001::17]:80

Decides to check :15
2016/02/20 22:18:32.560| ipcache.cc(1039) ipcacheCycleAddr: ipcacheCycleAddr: security.ubuntu.com now at [2001:67c:1562::15] (2 of 20)
2016/02/20 22:18:32.560| AsyncCall.cc(85) ScheduleCall: ConnOpener.cc(132) will call fwdConnectDoneWrapper(local=[::] remote=[2001:67c:1360:8001::17]:80 flags=1, errno=101, flag=-8, data=0x555c4de451b8) [call23]
(...)

last ipv6 one, starting ipv4:
2016/02/20 22:18:32.568| ipcache.cc(1075) ipcacheMarkBadAddr: ipcacheMarkBadAddr: security.ubuntu.com [2001:67c:1560:8001::11]:80
2016/02/20 22:18:32.568| ipcache.cc(1039) ipcacheCycleAddr: ipcacheCycleAddr: security.ubuntu.com now at 91.189.91.13 (11 of 20)
2016/02/20 22:18:32.568| AsyncCall.cc(85) ScheduleCall: ConnOpener.cc(132) will call fwdConnectDoneWrapper(local=[::] remote=[2001:67c:1560:8001::11]:80 flags=1, errno=101, flag=-8, data=0x555c4de451b8) [call68]
(...)

I don't see it trying the ipv4 address, just considering it, but no connection attempt:
2016/02/20 22:18:32.577| ConnOpener.cc(346) connect: local=[::] remote=[2001:67c:1562::15]:80 flags=1: * - ERR tried too many times already.
2016/02/20 22:18:32.577| ipcache.cc(1039) ipcacheCycleAddr: ipcacheCycleAddr: security.ubuntu.com now at 91.189.91.15 (13 of 20)
2016/02/20 22:18:32.577| AsyncCall.cc(85) ScheduleCall: ConnOpener.cc(132) will call fwdConnectDoneWrapper(local=[::] remote=[2001:67c:1562::15]:80 flags=1, errno=101, flag=-8, data=0x555c4de451b8) [call83]

That's how far I got.