Fix project image in Slack pipeline notifications

This commit fixes the project avatar images that are rendered in the
footer of Slack pipeline notifications.  Previously, the image URLs
provided to Slack were relative URLs; now they are absolute.

app/models/project_services/chat_message/pipeline_message.rb

@@ -68,7 +68,7 @@ module ChatMessage
         title_link: pipeline_url,
         fields: attachments_fields,
         footer: project.name,
-        footer_icon: project.avatar_url,
+        footer_icon: project.avatar_url(only_path: false),
         ts: finished_at
       }]
     end

changelogs/unreleased/66008-fix-project-image-in-slack-notifications.yml

+---
+title: Fix project avatar image in Slack pipeline notifications
+merge_request: 31788
+author:
+type: fixed

spec/models/project_services/chat_message/pipeline_message_spec.rb

@@ -42,9 +42,9 @@ describe ChatMessage::PipelineMessage do
 
   before do
     test_commit = double("A test commit", committer: args[:user], title: "A test commit message")
-    test_project = double("A test project",
-                          commit_by: test_commit, name: args[:project][:name],
-                          web_url: args[:project][:web_url], avatar_url: args[:project][:avatar_url])
+    test_project = double("A test project", commit_by: test_commit, name: args[:project][:name], web_url: args[:project][:web_url])
+    allow(test_project).to receive(:avatar_url).with(no_args).and_return("/avatar")
+    allow(test_project).to receive(:avatar_url).with(only_path: false).and_return(args[:project][:avatar_url])
     allow(Project).to receive(:find) { test_project }
 
     test_pipeline = double("A test pipeline", has_yaml_errors?: has_yaml_errors,