gitlab-ci: use barebones base image for easy auditing

F-Droid setups here deliberately use simple base images like
alpine so we can easily check whether they are all free
software, like the MapBox issues.  I have done quite a bit of
profiling in the past of docker images like curlimages vs
installing.  For short jobs like this, there has never been a
clear winner.  When curlimages is in the runner's docker
cache, it can be a bit faster than installing the package, but
images like alpine are much more likely to be in the docker
cache, making them on average faster.

this reverts a change in !11503.

.gitlab-ci.yml

@@ -171,7 +171,7 @@ trigger-issuebot:
   stage: prepare
   needs: []
   rules: *app_verification_rules
-  image: curlimages/curl:latest
+  image: alpine
   variables:
     GIT_DEPTH: "1"
   artifacts:
@@ -183,6 +183,7 @@ trigger-issuebot:
   script:
     - mkdir logs
     - export | grep -F CI_ | grep -vFi -e password -e token > logs/export.txt
+    - apk add --no-cache bash curl
     - ./tools/trigger-issuebot
 
 schedule-issuebot: