add pipeline for building docker images

The pipeline contains two different build jobs, building a standalone or a cms/cvmfs version. In addition it contains a job for checking which files have been modified, enabling to trigger the build of the cms/cvmfs version that was changed. Only reason for there to be a separate script `check.sh` and not just a multi line script is because I didn't get the multi line syntax to work with a for loop

```
script:
  - >
```

If you know how to fix it we can transfer the code in `check.sh` to the pipeline!

-----------------------------

The two build jobs:

* A standalone build is triggered when the variables `CMSSW_VERSION`, `SCRAM_ARCH` and `BASE_IMAGE` is passed, could be passed with a webhook.

* A cms/cvmfs version is built when there has been a change in the directory containing the Dockerfile corresponding to the image. Since there is only one job but five directories, before building, the job has to set the IMAGE_NAME variable to indicate which image to build (cc7-cms/cvmfs, slc6-cms/cvmfs, slc5-cms). This is set with the job `check changed files`

The file `check.sh` calls 

```
files=$(git diff-tree --no-commit-id --name-only -r $CI_COMMIT_SHA)
```

which returns a list of all files that has been modified. The directory of the file is checked and the pipeline is triggered again, this time with the variable IMAGE_NAME set to the directory name. (The reason why the job tags docker is because the runner has git installed).

See merge request cms-cloud/cmssw-docker!12

.gitlab-ci.yml

+stages:
+  - check
+  - build
+  
+before_script:
+    - export DATE=$(date +"%Y-%m-%d")
+
+variables:
+  COMMIT_HASH: $CI_COMMIT_SHORT_SHA
+
+check changed files:
+  stage: check
+  only:
+    changes: [cc7-cms/*, cc7-cvmfs/*, slc6-cms/*, slc6-cvmfs/*, slc5-cms/*]
+  except:
+    - pipelines
+  tags:
+    - docker
+  script:
+    - ./check.sh
+
+build standalone:
+  stage: build
+  only:
+    variables:
+      - $CMSSW_VERSION && $SCRAM_ARCH && $BASE_IMAGE
+  variables:
+    REPO_NAME: standalone
+    IMAGE_NAME: cmssw
+  tags:
+    - docker-privileged-xl
+  image:
+    name: gcr.io/kaniko-project/executor:debug
+    entrypoint: [""]
+  script:
+    - echo "Building standalone image $BASE_IMAGE with $CMSSW_VERSION and $SCRAM_ARCH"
+    - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /kaniko/.docker/config.json
+    - /kaniko/executor --context $CI_PROJECT_DIR/$REPO_NAME --dockerfile $CI_PROJECT_DIR/$REPO_NAME/Dockerfile --destination $CI_REGISTRY_IMAGE/$IMAGE_NAME:$CMSSW_VERSION-$DATE-$COMMIT_HASH --build-arg=BUILD_DATE="$DATE" --build-arg=VERSION="$DATE" --build-arg=VCS_URL="$CI_REPOSITORY_URL" --build-arg=VCS_REF="$COMMIT_HASH" --build-arg BASEIMAGE="clelange/$BASE_IMAGE-cms:latest" --build-arg SCRAM_ARCH="$SCRAM_ARCH" --build-arg CMSSW_VERSION="$CMSSW_VERSION"
+
+build cms cvmfs:
+  stage: build
+  only:
+    variables:
+      - $CI_PIPELINE_SOURCE == "pipeline" && $IMAGE_NAME
+  image:
+    name: gcr.io/kaniko-project/executor:debug
+    entrypoint: [""]
+  script:
+    - echo "Building image for $IMAGE_NAME"
+    - echo "{\"auths\":{\"$CI_REGISTRY\":{\"username\":\"$CI_REGISTRY_USER\",\"password\":\"$CI_REGISTRY_PASSWORD\"}}}" > /kaniko/.docker/config.json
+    - /kaniko/executor --context $CI_PROJECT_DIR/$IMAGE_NAME --dockerfile $CI_PROJECT_DIR/$IMAGE_NAME/Dockerfile --destination $CI_REGISTRY_IMAGE/$IMAGE_NAME:$DATE-$COMMIT_HASH --destination $CI_REGISTRY_IMAGE/$IMAGE_NAME:latest --build-arg=BUILD_DATE="$DATE" --build-arg=VERSION="$DATE" --build-arg=VCS_URL="$CI_REPOSITORY_URL" --build-arg=VCS_REF="$COMMIT_HASH"

check.sh

+#!/bin/bash
+
+files=$(git diff-tree --no-commit-id --name-only -r $CI_COMMIT_SHA)
+for file in $files; do
+  dir=${file%/*}
+
+  curl --request POST \
+  --form token=$CI_JOB_TOKEN \
+  --form ref=$CI_DEFAULT_BRANCH \
+  --form "variables[IMAGE_NAME]=$dir" \
+  https://gitlab.cern.ch/api/v4/projects/$CI_PROJECT_ID/trigger/pipeline
+done