Fix the Moore CI docs pipeline

Cherry-picked the relevant commits from !3422 (merged) as requested by @jonrob.

With help from @jonrob and @clemenci, I've modified the .gitlab-ci.yml configuration so that the pipeline can work out what branch is being targeted and also to which nightly build slot that would correspond.

The latter should fix the docs again and keep them alive on all protected branches (they were hard-coded to look at lhcb-master, but of course we're working on 2024-patches and the two have diverged).

The former should speed up the formatting check, as currently it is checking every file that differs between the branch and master for correct formatting - now it should just check only those that differ w.r.t the given target branch.

.gitlab-ci.yml

@@ -15,15 +15,15 @@ stages:
   - deploy
 
 variables:
-  TARGET_BRANCH: master
   # Parameters for downloading nightlies and building Moore
   BINARY_TAG: x86_64_v2-el9-gcc13-opt
   NO_LBLOGIN: "1" # prevent lbdocker containers to start LbLogin/LbEnv
-  LB_NIGHTLY_SLOT: lhcb-master
 
 check-copyright:
   stage: check
   image: gitlab-registry.cern.ch/ci-tools/ci-worker:cc7
+  before_script:
+    - TARGET_BRANCH=${CI_MERGE_REQUEST_TARGET_BRANCH_NAME:-${CI_COMMIT_BRANCH}}
   script:
     - curl -o lb-check-copyright "https://gitlab.cern.ch/lhcb-core/LbDevTools/raw/master/LbDevTools/SourceTools.py?inline=false"
     - python lb-check-copyright origin/${TARGET_BRANCH} --exclude lhcbproject.yml
@@ -33,6 +33,8 @@ check-formatting:
   tags:
     - cvmfs
   image: gitlab-registry.cern.ch/ci-tools/ci-worker:cc7
+  before_script:
+    - TARGET_BRANCH=${CI_MERGE_REQUEST_TARGET_BRANCH_NAME:-${CI_COMMIT_BRANCH}}
   script:
     - . /cvmfs/lhcb.cern.ch/lib/LbEnv.sh
     - if [ ! -e .clang-format ] ; then
@@ -62,6 +64,8 @@ build:
   image: gitlab-registry.cern.ch/lhcb-docker/os-base/alma9-devel:latest
   tags:
     - cvmfs
+  before_script:
+    - LB_NIGHTLY_SLOT="lhcb-${CI_MERGE_REQUEST_TARGET_BRANCH_NAME:-${CI_COMMIT_BRANCH}}"
   script:
     - . /cvmfs/lhcb.cern.ch/lib/LbEnv.sh
     - build_path=/cvmfs/lhcbdev.cern.ch/nightlies/${LB_NIGHTLY_SLOT}/latest
@@ -93,12 +97,13 @@ build-docs:
     # As the previous stage is allowed to fail, skip this stage if the previous
     # did not complete successfully (i.e. did not create the `run` symlink)
     - test -L run || { echo "WARNING build job failed, stopping"; exit 77; }
+    - TARGET_BRANCH=${CI_MERGE_REQUEST_TARGET_BRANCH_NAME:-${CI_COMMIT_BRANCH}}
   script:
     # First try producing just the graphs (gaudirun.py). If that fails, Moore
     # is likely incompatible with the last nightly, so fail with a warning.
     - ./run make -C doc graphs || { echo "WARNING build is incompatible with last nightly, stopping"; exit 77; }
     - reasons=
-    - ./run python doc/make_functor_docs.py "${CI_COMMIT_SHA:0:8}" "master" > doc/selection/thor_functors_reference.generated.rst || reasons+='ERROR failed functor docs build\n'
+    - ./run python doc/make_functor_docs.py "${CI_COMMIT_SHA:0:8}" "${TARGET_BRANCH}" > doc/selection/thor_functors_reference.generated.rst || reasons+='ERROR failed functor docs build\n'
     - ./run make -C doc linkcheck || reasons+='ERROR failed link check\n'
     - ./run make -C doc html || reasons+='ERROR failed html generation\n'
     - if [ -n "$reasons" ]; then echo -e $reasons; exit 1; fi