Run sphinx-build over all cores

Use `sphinx-build -j` and `nproc` to distribute collecting and compiling
the documentation over all cores.

.gitlab-ci.yml

@@ -55,7 +55,7 @@ docs:
   script:
     - echo "Building current docs"
     - env -u GITLAB_CI make doctest
-    - env -u GITLAB_CI sphinx-build -M html "." "_build" -W
+    - env -u GITLAB_CI sphinx-build -M html "." "_build" -W -j $(nproc)
     - echo "Checking if there are changes"
     - ./generate-apidocs.sh
     - git diff --exit-code

resources/entrypoint.sh

@@ -2,7 +2,7 @@
 
 set -e
 
-sphinx-build -b html /usr/src/app/docs/ /concrexit/docs/
+sphinx-build -b html /usr/src/app/docs/ /concrexit/docs/ -j $(nproc)
 
 until psql -h "$DJANGO_POSTGRES_HOST" -U "postgres" -c '\l'; do
     >&2 echo "PostgreSQL is unavailable: Sleeping"

resources/entrypoint_production.sh

@@ -3,7 +3,7 @@
 set -e
 
 >&2 echo "Creating Sphinx documentation"
-sphinx-build -b html /usr/src/app/docs/ /concrexit/docs/
+sphinx-build -b html /usr/src/app/docs/ /concrexit/docs/ -j $(nproc)
 
 until psql -h "$DJANGO_POSTGRES_HOST" -U "postgres" -c '\l' $POSTGRES_DB; do
     >&2 echo "PostgreSQL is unavailable: Sleeping"