thodg/libgit2

diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 73f8454..fc95315 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -11,8 +11,8 @@ jobs:
     strategy:
       matrix:
         container:
-        - { name: xenial, base: ubuntu:xenial }
-        - { name: bionic, base: ubuntu:bionic }
+        - { name: xenial, base: 'ubuntu:xenial' }
+        - { name: bionic, base: 'ubuntu:bionic' }
     env:
       docker-config-path: azure-pipelines/docker
     name: Build docker image