Add instructions for running PR-Agent as a GitLab pipeline in gitlab.md

docs/docs/installation/gitlab.md

@@ -1,3 +1,42 @@
+## Run as a GitLab Pipeline
+You can use a pre-built Action Docker image to run PR-Agent as a GitLab pipeline. This is a simple way to get started with PR-Agent without setting up your own server.
+
+(1) Add the following file to your repository under `.gitlab-ci.yml`:
+```yaml
+stages:
+  - pr_agent
+
+pr_agent_job:
+  stage: pr_agent
+  image: 
+    name: codiumai/pr-agent:latest
+    entrypoint: [""]
+  script:
+    - cd /app
+    - echo "Running PR Agent action step"
+    - export MR_URL="$CI_MERGE_REQUEST_PROJECT_URL/merge_requests/$CI_MERGE_REQUEST_IID"
+    - echo "MR_URL=$MR_URL"
+    - export gitlab__PERSONAL_ACCESS_TOKEN=$GITLAB_PERSONAL_ACCESS_TOKEN 
+    - export config__git_provider="gitlab"
+    - export openai__key=$OPENAI_KEY
+    - python -m pr_agent.cli --pr_url="$MR_URL" describe
+    - python -m pr_agent.cli --pr_url="$MR_URL" review
+    - python -m pr_agent.cli --pr_url="$MR_URL" improve
+  rules:
+    - if: '$CI_PIPELINE_SOURCE == "merge_request_event" && $CI_MERGE_REQUEST_EVENT_TYPE == "merge_request" && $CI_MERGE_REQUEST_STATE == "opened"'
+```
+Note that this script will run PR-Agent on every new merge request. You can modify the `rules` section to run PR-Agent on different events.
+You can also modify the `script` section to run different PR-Agent commands, or with different parameters by exporting different environment variables.
+
+
+(2) Add the following variables to your GitLab repository (CI/CD -> Variables):
+- `GITLAB_PERSONAL_ACCESS_TOKEN`: Your GitLab personal access token.
+- `OPENAI_KEY`: Your OpenAI key.
+
+Note that if your base branches are not protected, don't set the variables as `protected`, since the pipeline will not have access to them.
+
+
+
 ## Run a GitLab webhook server
 
 1. From the GitLab workspace or group, create an access token. Enable the "api" scope only.