Support git strategy with kubernetes executor

What does this MR do?

This MR allows the configuration of persistent volumes for the /builds dir.

Why was this MR needed?

This MR was needed to allow the support of the GIT_STRATEGY: fetch

What's the best way to test this MR?

Configurations

volume.yml

apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv-1
spec:
  capacity:
    storage: 5Gi
  volumeMode: Filesystem
  storageClassName: local-storage
  accessModes:
    - ReadWriteOnce
  hostPath:
    path: /mnt/pv-data/pv-1

pvc1.yml

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: pvc-1
  namespace: default
spec:
  storageClassName: local-storage
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 3Gi

.gitlab-ci.yml

variables:
  GIT_STRATEGY: fetch
  DURATION: 10
  FF_USE_LEGACY_KUBERNETES_EXECUTION_STRATEGY: "false"
job:
  script:
  - 'for i in $(seq 1 $DURATION); do echo $(date); sleep 1; done'
  - echo "done"

config.toml

[[runners]]
  name = "kubernetes"
  url = "https://gitlab.com/"
  token = "TOKEN_HERE"
  executor = "kubernetes"
  builds_dir = "/root_builds"
  [runners.kubernetes]
    image = "alpine:3.11"
    privileged = true
    namespace = "default"
    [[runners.kubernetes.volumes.pvc]]
      name = "pvc-1"
      mount_path = "/root_builds"

Steps

1. Create PV with the following command

kubectl create -f PATH/TO/volume.yml

Following message validate the PV creation

persistentvolume/pv-1 created

2. Create PVC with the following command

kubectl create -f PATH/TO/pvc1.yml

Following message validate the PVC creation

persistentvolumeclaim/pvc-1 created

3. Run the pipeline and wait until it is done. The log file will look as follow log
4. Connect to minikube node and check if the repo are still presents

minikube ssh
ls -alF /mnt/pv-data/pv-1/

What are the relevant issue numbers?

closes: #3847 (closed)