[actions] update workbench docker version to v0.2.8 (#8)

Co-authored-by: googlieyes <183037054+googlieyes@users.noreply.github.com>

Author: carpentries-bot

.github/workbench-docker-version.txt

@@ -1 +1 @@
-v0.2.7
+v0.2.8