Buildx image building (for multiarch support) requires us to set up CA
certificates that are trusted by the image for access to the
registr(y|ies). The buildx image switched to alpine at some point which
doesn't include the update-ca-certificates command we were depending on
for this by default. Install that package so that we can
update-ca-certificates and trust the registr(y|ies).
Note that there may be a better way via buildkitd.toml configuration
files that allow us to configure certs upfront and have the image set
them up for us. This is documented in a todo as a future action we can
investigate.
Change-Id: Ied0b8e81ed6a7a15ea2def26b85b933223cb42e6