Consider a slimmer base image (e.g. distroless)

**Describe the problem/challenge you have**
We'd like to ship kapp-controller with the minimum necessary bits. We're currently using [photon as our base image](https://github.com/vmware-tanzu/carvel-kapp-controller/blob/develop/Dockerfile#L1). Is there a better (i.e. slimmer) option?

**Describe the solution you'd like**
Investigate alternative options (such as [distroless](https://github.com/GoogleContainerTools/distroless)) and determine if we should consider making a switch. If the changes appear to be simple, feel free to make the necessary changes.

**Anything else you would like to add:**
- similar issue in secretgen-controller: vmware-tanzu/carvel-secretgen-controller#40

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