Fix arch build

.github/actions/build-arch/entrypoint.sh

@@ -1,15 +1,22 @@
 #!/bin/bash
 
 set -e
-
 cd linux
+
+# Create user "docker" because makepkg refuses to run as root.
+id docker || (echo creating user docker; useradd docker)
 chown -R docker:docker .
 
-# Reset PKGEXT to it's default (it's different in the build-container)
-PKGEXT=".pkg.tar.xz" su docker -c makepkg
+# Set PKGDEST to a location where user "docker" can write.
+export PKGDEST="."
+# Reset PKGEXT to it's default (it's different in the build-container).
+export PKGEXT=".pkg.tar.xz"
 
-ls -lah
+# Build package.
+su docker -c makepkg
 
+# Check result and prepare for uploading.
+ls -lah
 pkgname=$(ls *.pkg.tar*)
 mv $pkgname ../$pkgname
-echo ::set-output name=filename::$pkgname
+echo ::set-output name=filename::$pkgname