CI: Allow running in a container.

Add a parameter of the pr-gate-build-job template to specify a
container image URL.  If the value is not '' (default), then the
jobs will be run inside a container based on that image.

Signed-off-by: Oliver Steffen <osteffen@redhat.com>
Reviewed-by: Michael Kubacki <michael.kubacki@microsoft.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chris Fernald <chfernal@microsoft.com>

.azurepipelines/templates/pr-gate-build-job.yml

@@ -14,6 +14,7 @@ parameters:
   arch_list: ''
   extra_install_step: []
   usePythonVersion: ''
+  container: ''
 
 # Build step
 jobs:
@@ -72,6 +73,9 @@ jobs:
   pool:
     vmImage: ${{ parameters.vm_image }}
 
+  ${{ if not(eq(parameters.container, '')) }}:
+    container: ${{ parameters.container }}
+
   steps:
   - template: pr-gate-steps.yml
     parameters: