Try buster with APT.

.gitlab-ci.yml

-image: 3.7.7-buster:latest
+image: python:3.7-slim-buster
 
 stages:
   - bundle
@@ -16,7 +16,7 @@ cache:
 bundle_and_test:
   stage: bundle
   before_script:
-    - apk add --update npm
+    - apt install nodejs npm
     - npm install -g @apidevtools/swagger-cli
 
   script:
@@ -40,7 +40,7 @@ test_api:
 pages:
   stage: deploy
   before_script:
-    - apk add --update npm
+    - apt install nodejs npm
     - npm install swagger-ui-dist@3.22.1
   script:
     - cp -rp node_modules/swagger-ui-dist/* ./public