Update README.md

Author: jugatsu

README.md

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+# Description
+Copy-Paste Ready™ how-to for using private [Gitlab Container Registry](https://docs.gitlab.com/ce/user/project/container_registry.html) with [Kubernetes](https://kubernetes.io).
+
+#### Create env file with credentials
+
+```bash
+cat <<EOF >dockercfg.env
+export DOCKER_REGISTRY_SERVER=<registry_url>
+export DOCKER_USER=<username>
+export DOCKER_PASSWORD=<password>
+export DOCKER_EMAIL=<email>
+export DOCKER_IMAGE_PATH=<image_path_with_tag>
+EOF
+```
+
+#### Edit accordingly
+See `dockercfg.env.example`
+
+#### Export additional variables
+```bash
+export REGISTRY_NAME=`echo $DOCKER_REGISTRY_SERVER | sed -e 's/^http:\/\///g' -e 's/^https:\/\///g'`
+export DOCKER_IMAGE_FULL_PATH=$REGISTRY_NAME/$DOCKER_IMAGE_PATH
+```
+
+#### Create secret in the Kubernetes Cluster
+```bash
+kubectl create secret docker-registry gitlab-registry \
+  --docker-server=$DOCKER_REGISTRY_SERVER \
+  --docker-username=$DOCKER_USER \
+  --docker-password=$DOCKER_PASSWORD \
+  --docker-email=$DOCKER_EMAIL
+```
+
+#### Test
+```bash
+cat <<EOF | kubectl apply -f -
+apiVersion: v1
+kind: Pod
+metadata:
+  name: foo
+spec:
+  containers:
+    - name: foo
+      image: $DOCKER_IMAGE_FULL_PATH
+      imagePullPolicy: Always
+  imagePullSecrets:
+    - name: gitlab-registry
+EOF
+```
+
+#### Check if the image was pulled successfully
+```bash
+kubectl describe po/foo | grep -i pull
+```
+
+### Cleanup
+#### Remove pod
+```bash
+kubectl delete po/foo
+```
+#### Remove file with environment variables
+```bash
+rm dockercfg.env
+```
+#### Unset environment variables
+```bash
+unset DOCKER_REGISTRY_SERVER
+unset DOCKER_USER
+unset DOCKER_EMAIL
+unset DOCKER_PASSWORD
+unset DOCKER_IMAGE_PATH
+unset REGISTRY_NAME
+unset DOCKER_IMAGE_FULL_PATH
+```