feat: Supports extend of entrypoint

[k-kinzal]

We may need helm plugins when using helm.
It is very troublesome to recreate containers.

So we need a way to install A with entrypoint.

proposal

aliases.yaml

/usr/local/bin/helm
  image: chatwork/helm
  tag: 2.11.0
  volumes:
  - $HOME/.kube:/root/.kube
  - $HOME/.helm:/root/.helm
  - $PWD:/helm
  workdir: /helm
  entrypoint: |
    #!/bin/sh
    if [ -f $(helm home)/plugins/helm-import ]; then
      helm plugin install https://github.com/k-kinzal/helm-import
    if
    helm "$@"
  dependencies:
  - /bin/bash
  - /bin/git

script

$ docker run ... -v $HOME/.aliases/entrypoints/xxx:/xxx --entrypoint /xxx chatwork/helm:${HELM_VERSION:-2.11.0}

We think that we should be able to extend entrypoint like this.