Remove docker build from PRs (#573)

.github/workflows/docker.yml

@@ -6,15 +6,6 @@ on:
       - '[0-9]+.[0-9]+.[0-9]+'
   release:
     types: [published]
-  pull_request:
-    branches:
-      - master
-    paths:
-      - 'Dockerfile'
-      - 'Cargo.toml'
-      - 'Cargo.lock'
-      - 'contributing.Dockerfile'
-      - '.github/workflows/docker.yml'
 
 jobs:
   push_to_registry: