[devdocsgjs] Use actual pipes for parallelized build

[Bartłomiej Piotrowski <bpiotrowski src gnome org>]

commit 91a0e862dfbb5fbc514422a0ddbd70f99a873f0f
Author: Bartłomiej Piotrowski <bpiotrowski gnome org>
Date:   Wed Apr 6 09:27:44 2022 +0200

    Use actual pipes for parallelized build

 Dockerfile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/Dockerfile b/Dockerfile
index 4b4c90f6..e23adb16 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -112,7 +112,7 @@ RUN echo adw1 appindicator301 appstreamglib10 atk10 atspi20 cairo10 \
         cally8 clutter8 clutterx118 cogl8 coglpango8 meta8 \
         cally9 clutter9 cogl9 coglpango9 meta9 \
         cally10 clutter10 cogl10 coglpango10 meta10 \
-        || tr ' ' '\n' || xargs -L1 -P4 bundle exec thor docs:generate --force
+        | tr ' ' '\n' | xargs -L1 -P$(nproc) bundle exec thor docs:generate --force
 
 # We deploy in ruby:2.7.3-alpine for size
 #