Configure concurrency setting for actions

We can set a concurrency setting for each of our actions so that pushing
to a branch cancels in-progress runs without conflicting with runs on
other branches

https://docs.github.com/en/actions/using-jobs/using-concurrency#example-only-cancel-in-progress-jobs-or-runs-for-the-current-workflow

.github/workflows/django.yml

@@ -8,6 +8,10 @@ on:
     branches:
       - master
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   docs:
     name: Run tests

.github/workflows/docker.yml

@@ -9,6 +9,10 @@ on:
     branches:
       - "master"
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   docker:
     runs-on: ubuntu-20.04

.github/workflows/sphinx.yml

@@ -7,9 +7,8 @@ on:
     branches:
       - master
 
-# Allow one concurrent deployment
 concurrency:
-  group: "pages"
+  group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: true
 
 jobs:
@@ -58,4 +57,3 @@ jobs:
       - name: Deploy to GitHub Pages
         id: deployment
         uses: actions/deploy-pages@v2
-