ci: Add 'check yamls' job in (new) 'check input' stage

Add a new stage 'check input' to our CI configuration to check whether
our input files contain errors.
The first check is for the YAML files whether they contain errors or
warnings according to 'yamllint'.

Add 'yamllint' config file to not consider too long lines as errors.
And fix the errors/warnings in 'salsa-ci.yml' itself.

Apparently job names can contain space, so rename 'build-yamls' to
'build yamls'.

debian/salsa-ci.yml

@@ -1,4 +1,7 @@
+---
 stages:
+  # Garbage in is garbage out, so verify our input
+  - check input
   - build
 
 variables:
@@ -7,7 +10,16 @@ variables:
   SALSA_CI_IMAGES: registry.salsa.debian.org/salsa-ci-team/pipeline
   BASE_CI_IMAGES: ${SALSA_CI_IMAGES}/base
 
-build-yamls:
+check yamls:
+  stage: check input
+  image: $BASE_CI_IMAGES:unstable
+  dependencies: []
+  script:
+    - apt-get update && apt-get upgrade -y
+    - apt-get install -y yamllint
+    - yamllint -c debian/yamllint.yml .
+
+build yamls:
   stage: build
   image: $BASE_CI_IMAGES:unstable
   dependencies: []
@@ -20,4 +32,3 @@ build-yamls:
   artifacts:
     paths:
       - build/
-