Update ca-certificates.crt in blackbox exporter docker image

[nfriday]

The latest tag of the blackbox exporter docker image has a somewhat outdated version of ca-certificates.crt; and we are starting to see some probes fail due to untrusted root CA's as newer certificates are issued:
-rw-r--r-- 1 root root 216591 Dec 3 05:01 ca-certificates.crt

I can workaround this by pulling in or mounting an updated version from debian or elsewhere, but though this ought to be updated.