ci: Add 'shellcheck' job to 'check input' stage

Check all the scripts, under 'rootfs/etc/initramfs-tools' (for now) with
'shellcheck'.
A couple of checks are excluded of which SC1090 and SC1091 will remain.
But SC2086 and especially SC2154 should be fixed! And when they are
fixed in the current code, those should be enabled again for checking.

Also rename 'check yamls' job to 'yamllint', so that's directly clear
which tool is being used.

debian/salsa-ci.yml

@@ -10,7 +10,7 @@ variables:
   SALSA_CI_IMAGES: registry.salsa.debian.org/salsa-ci-team/pipeline
   BASE_CI_IMAGES: ${SALSA_CI_IMAGES}/base
 
-check yamls:
+yamllint:
   stage: check input
   image: $BASE_CI_IMAGES:unstable
   dependencies: []
@@ -19,6 +19,15 @@ check yamls:
     - apt-get install -y yamllint
     - yamllint -c debian/yamllint.yml .
 
+shellcheck:
+  stage: check input
+  image: $BASE_CI_IMAGES:unstable
+  dependencies: []
+  script:
+    - apt-get update && apt-get upgrade -y
+    - apt-get install -y shellcheck
+    - shellcheck -e SC1090,SC1091,SC2086,SC2154 -s dash $(find rootfs/etc/initramfs-tools -type f -executable  | xargs grep -l '^#!/bin/sh')
+
 build yamls:
   stage: build
   image: $BASE_CI_IMAGES:unstable