ci: go back to sphinx-build-env to enable PDF build

.gitlab-ci.yml

-image: beagle/debian-build:latest
+image: beagle/sphinx-build-env:latest
 
 variables:
   GIT_SUBMODULE_STRATEGY: recursive
@@ -6,6 +6,7 @@ variables:
 cache:
   paths:
   - .venv
+  - .cache
 
 build:
   stage: build
@@ -18,7 +19,6 @@ build:
     paths:
     - public/$TARGET
   before_script:
-    - apt update && apt install -y python3-venv doxygen
     - source ./venv-build-env.sh
   script:
     - ./gitlab-build.sh $TARGET

gitlab-build.sh

@@ -48,13 +48,13 @@ HERE
 
 		echo "**** make html ****"
 		# Build HTML
-		make html -j$(nproc) BUILDDIR=public
+		make html BUILDDIR=public
 	fi
 
 	if [ "x$1" == "xpdf" ]; then
 		echo "**** make latexpdf ****"
 		# Build, optimize, and serve PDF
-		make latexpdf -j$(nproc) BUILDDIR=public
+		make latexpdf BUILDDIR=public
 
 		echo "**** pdfcpu ****"
 		pdfcpu version