Reset ENTRYPOINT, run kaniko in CMD block

[bittner]

Actual behavior

Kaniko executor as ENTRYPOINT makes it hard to use the image for CI pipelines (e.g. GitLab). You have to reconfigure the entrypoint to /bin/sh or reset its value using ENTRYPOINT [].

# Example .gitlab-ci.yml (note the `entrypoint` below), taken from 
# https://docs.gitlab.com/ee/ci/docker/using_kaniko.html#building-a-docker-image-with-kaniko
build:
  stage: build
  image:
    name: gcr.io/kaniko-project/executor:debug
    entrypoint: [""]
  script:
  - mkdir -p /kaniko/.docker
  - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /kaniko/.docker/config.json
  - /kaniko/executor --context $CI_PROJECT_DIR --dockerfile $CI_PROJECT_DIR/Dockerfile --destination $CI_REGISTRY_IMAGE:$CI_COMMIT_TAG

Expected behavior

The image should be usable with CI systems such as GitLab CI or GitHub Actions out-of-the-box (e.g. use an entrypoint script, fall back to /bin/sh when that is overridden, only use CMD to run kaniko for convenience or don't run kaniko by default at all).

A resulting CI setup would be simpler, e.g.

build:
  stage: build
  image: gcr.io/kaniko-project/executor:debug
  script:
  - ...

To Reproduce

Steps to reproduce the behavior:

1. Use the debug image with GitLab or GHA (e.g. image: gcr.io/kaniko-project/executor:default) without reconfiguring the entrypoint
2. Run the pipeline -- it will fail
3. Remove ENTRYPOINT and CMD (or set ENTRYPOINT [])
4. Run the pipeline -- it will succeed

Triage Notes for the Maintainers

Description	Yes/No
Please check if this a new feature you are proposing
Please check if the build works in docker but not in kaniko
Please check if this error is seen when you use --cache flag
Please check if your dockerfile is a multistage dockerfile