Use nextflow image in ci.yml

.gitlab-ci.yml

-image: python:3.9-slim-buster
-
 stages:
   - qc
   - test
 
 qc_1:
   stage: qc
-
+  image: python:3.9-slim-buster
   before_script:
     - pip install -r requirements.txt
     - pip install -r requirements_dev.txt
     - pip install -e .
-    
   script:
     - flake8 src/ setup.py
     - coverage run --source src/ -m pytest
@@ -20,16 +17,7 @@ qc_1:
 
 test_1:
   stage: test
-
-  before_script:
-    - pip install -r requirements.txt
-    - pip install -r requirements_dev.txt
-    - pip install -e .
-    - apt update -y
-    - apt install wget -y
-    - wget -qO- https://get.nextflow.io | bash
-    - nextflow self-update
-
+  image: nextflow/nexflow:21.10.3
   script:
     - nextflow main.nf