run build on ubuntu-22.04 #27

mlocati/docker-php-extension-installer#1053 (comment)
suggests that it might fix our build problems.

Author: splitbrain

.github/workflows/docker.yml

@@ -7,7 +7,7 @@ on:
 jobs:
   buildmatrix:
     name: Create Build Matrix
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     outputs:
       matrix: ${{ steps.set-matrix.outputs.matrix }}
     steps: