azure: coverity: fix invalid syntax for Docker image

In commit bbc0b20b (azure: fix Coverity's build due to wrong container
name, 2019-08-02), Coverity builds were fixed to use the correct
container names. Unfortunately, the "fix" completely broke our Coverity
builds due to using wrong syntax for the Docker task. Let's fix this by
using "imageName" instead of the Docker dict.

azure-pipelines/coverity.yml

@@ -11,9 +11,7 @@ jobs:
     displayName: Build
     inputs:
       action: 'Run an image'
-      docker:
-        image: xenial
-        base: xenial
+      imageName: libgit2/xenial
       volumes: |
        $(Build.SourcesDirectory):/home/libgit2/source
        $(Build.BinariesDirectory):/home/libgit2/build