chore(build): Install Helm3 Binary to Rosco Container Image

[evansb]

Install Helm 3 Binary alongside Helm 2.

Part of spinnaker/spinnaker#5474

Test Plan

$ docker build -t compile -f Dockerfile.compile .
$ docker build -t slim -f Dockerfile.slim .
$ docker run --rm slim helm version
Client: &version.Version{SemVer:"v2.16.3", GitCommit:"1ee0254c86d4ed6887327dabed7aa7da29d7eb0d", GitTreeState:"clean"}
Error: Get http://localhost:8080/api/v1/namespaces/kube-system/pods?labelSelector=app%3Dhelm%2Cname%3Dtiller: dial tcp 127.0.0.1:8080: connect: connection refused
$ docker run --rm slim helm3 version
version.BuildInfo{Version:"v3.1.0", GitCommit:"b29d20baf09943e134c2fa5e1e1cab3bf93315fa", GitTreeState:"clean", GoVersion:"go1.13.7"}

[ethanfrogers]

👍