Extend Kaniko to support gitlab login through env vars.

Just playing around for the moment.

to get to run in Gitlab CI under a fairly standard docker setup (not kubernetes) you need something like this in your .gitlab-ci.yml file:

testing kaniko:
  stage: build
  image: opb2k/kaniko-gitlab
  script:
    - echo '{"credHelpers":{"my.gitlab.server:4567":"gitlab-login"}}' > /kaniko/.docker/config.json
    - 'kaniko-build --destination ${CI_REGISTRY_IMAGE}:latest --context $CI_PROJECT_DIR'

Few issues it would be nice to get tidied up:

1. need to use debug image in order to have sh available. This is needed because gitlab runners using the docker executor will normally set a shell. Need to try a runner config which won't try to use a shell not sure if this is possible.
2. have to set --context because can't configure gitlab to use simply /workspace. Could potentially overwrite the way the command is called in our Dockerfile, removing need for this step.
3. need something better for setting the docker config.json. Very ugly right now.

Could probably use a shell script to do some of this (given we have busybox installed anyway) which would leave the interface to use in the .gitlab-ci.yml file pretty lean.