Switch to using perl-tester:* images for testing.

Some of the older upstream `perl:*` images are based on Debian
"Stretch", which is past EOL and no longer has Apt updates available.

The `perl-tester:*` images, however, are all based on Debian "Buster",
which is more recent and is continuing to receive updates.

Author: bleargh45

.github/workflows/perl-ci.yml

@@ -77,7 +77,7 @@ jobs:
           - "5.8"
 
     container:
-      image: perl:${{ matrix.perl-version }}
+      image: perldocker/perl-tester:${{ matrix.perl-version }}
 
     steps:
       - name: Download tarball artifact