release/tools: use pkg settings in /etc/pkg/FreeBSD.conf in containers

[dch]

The FreeBSD-base url here needs to have the pkg+: prefix otherwise it doesn't work in our new CDN world. It seems simplest to just enable here, and let /etc/pkg/FreeBSD.conf specify everything else.

I don't know what the correct path for the file should be, but it seems using the same as /etc/pkg/FreeBSD.conf seems the least confusing.

This is what we advise users to do in that file:

# To disable a repository, instead of modifying or removing this file,
# create a /usr/local/etc/pkg/repos/FreeBSD.conf file, e.g.:
#
#   mkdir -p /usr/local/etc/pkg/repos
#   echo "FreeBSD-ports: { enabled: no }" > /usr/local/etc/pkg/repos/FreeBSD.conf
#   echo "FreeBSD-ports-kmods: { enabled: no }" >> /usr/local/etc/pkg/repos/FreeBSD.conf
#
# Note that the FreeBSD-base repository is disabled by default.
...

[ivy]

i'm not familiar enough with the OCI build to review this, but the change looks right: we shouldn't be duplicating information that's already in /etc/pkg/FreeBSD.conf. this would also fix downstream users (like me) who ship a modified FreeBSD.conf.