Dockerfile needs update to remove a couple of vulnerabilities from the image

[brutif]

kustomize v0.30 includes a couple of CVE's from Alpine, CVE-2022-42915, curl, 9.8 and CVE-2022-43680 expat, 7.5

This can be easily cleared up by changing
https://github.com/fluxcd/kustomize-controller/blob/main/Dockerfile#L34
to FROM alpine:3.16.2 which will pick up the fixes from Alpine.

[pjbgf]

The base image CVEs are fixed as part of our release process. We are planning for a new release to be made in the coming days, once that takes place the image will be patched.

xref: fluxcd/flux2#3282

[brutif]

great, thanks @pjbgf

[brutif]

seems cleared up in v0.31.0, so closing