docs: how to to run in gitlab ci

docs/GitlabCI.md

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+# Running dockertest in Gitlab CI
+
+## How to run dockertest on shared gitlab runners?
+
+You should add docker dind service to your job which starts in sibling container. 
+That means database will be available on host `docker`.   
+You app should be able to change db host through environment variable.
+
+Here is the simple example of `gitlab-ci.yml`:
+```yaml
+stages:
+  - test
+go-test:
+  stage: test
+  image: golang:1.15
+  services:
+    - docker:dind
+  variables:
+    DOCKER_HOST: tcp://docker:2375
+    DOCKER_DRIVER: overlay2
+    YOUR_APP_DB_HOST: docker
+  script:
+    - go test ./...
+```
+
+Plus in the `pool.Retry` method that checks for connection readiness,
+ you need to use `$YOUR_APP_DB_HOST` instead of localhost.
+
+
+## How to run dockertest on group(custom) gitlab runners?
+Gitlab runner can be run in docker executor mode to save compatibility with shared runners.    
+Here is the simple register command:
+```shell script
+gitlab-runner register -n \
+ --url https://gitlab.com/ \
+ --registration-token $YOUR_TOKEN \
+ --executor docker \
+ --description "My Docker Runner" \
+ --docker-image "docker:19.03.12" \
+ --docker-privileged
+```
+
+You only need to instruct docker dind to start with disabled tls.  
+Add variable `DOCKER_TLS_CERTDIR: ""` to `gitlab-ci.yml` above.
+It will tell docker daemon to start on 2375 port over http. 
+
+