Refactor Dockerfile; create k8s deployment manifests

[alexmt]

PR adds kubernetes manifests and refactors Dockerfile. Documentation is work in progress and will be updated in next PR.

To test on kubernetes an Argo CD application using YAML below and argocd-bot-secret:

kubectl create secret generic argocd-bot-secret \
  --from-literal=APP_ID=$APP_ID \
  --from-literal=GHE_HOST=$GHE_HOST \
  --from-literal=TOKEN=$TOKEN \
  --from-literal=ARGOCD_SERVER=$ARGOCD_SERVER \
  --from-literal=GITHUB_TOKEN=$GITHUB_TOKEN \
  --from-literal=GITHUB_REPO=$GITHUB_REPO \
  --from-literal=WEBHOOK_SECRET=$WEBHOOK_SECRET \
  --from-file=key.pem=$PRIVATE_KEY_PATH

apiVersion: argoproj.io/v1alpha1
kind: Application
metadata:
  name: argocd-bot
spec:
  destination:
    namespace: argocd
    server: https://kubernetes.default.svc
  project: default
  source:
    kustomize:
      images:
      - argocd-bot=alexmt/argocd-bot:latest
    path: deployment/base
    repoURL: https://github.com/alexmt/argocd-bot
    targetRevision: k8s-deployment

Sample app is deployed here: https://cd.apps.argoproj.io/applications/argocd-bot . Test PR: argoproj/argo-cd-hello-world-config#4

[marcb1]

I rebase this branch, will test on master and build a public docker image