Run CI using GitHub Actions

This is a basically identical port of the existing CircleCI jobs
to GitHub Actions.

The linkcheck job is configured separately since it runs on a cron
rather than on push/PR.

Refs <freedomofpress/securedrop-tooling#5>.

.github/workflows/ci.yml

@@ -0,0 +1,25 @@
+name: CI
+on: [push, pull_request]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    container: debian:bullseye
+    steps:
+      - uses: actions/checkout@v3
+      - name: Install dependencies
+        run: |
+          apt-get update && apt-get install --yes --no-install-recommends make python3-pip
+          pip install poetry==1.4.0
+          poetry install --no-ansi
+      - name: Run lint
+        run: |
+          make docs-lint
+
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Build production deployment
+        run: |
+          docker build --build-arg GIT_BRANCH=$(git rev-parse HEAD) --file deploy/Dockerfile .

.github/workflows/linkcheck.yml

@@ -0,0 +1,19 @@
+name: Link check
+on:
+  schedule:
+    - cron: '0 3 * * *'
+
+jobs:
+  linkcheck:
+    runs-on: ubuntu-latest
+    container: debian:bullseye
+    steps:
+      - uses: actions/checkout@v3
+      - name: Install dependencies
+        run: |
+          apt-get update && apt-get install --yes --no-install-recommends make python3-pip
+          pip install poetry==1.4.0
+          poetry install --no-ansi
+      - name: Run lint
+        run: |
+          make docs-linkcheck