[Followup] Change base images to `bci-micro`

[Oats87]

Follow up to https://github.com/rancher/system-agent/pull/169 as the images would not build due to the lack of zypper in the bci-micro image

[Oats87]

Validated this works:

ck-devbox:~/go/src/github.com/rancher/system-agent # docker run -it --rm --entrypoint /bin/sh registry.suse.com/bci/bci-micro:15.6
sh-4.4# openssl
sh: openssl: command not found
sh-4.4# exit
exit
ck-devbox:~/go/src/github.com/rancher/system-agent # docker run -it --rm --entrypoint /bin/sh rancher/system-agent:dev
sh-4.4# openssl
help:

Standard commands
asn1parse         ca                ciphers           cmp
cms               crl               crl2pkcs7         dgst
dhparam           dsa               dsaparam          ec
ecparam           enc               engine            errstr
<snip>

sh-4.4# zypper
sh: zypper: command not found