Use docker container for link check

.github/workflows/ci.yml

@@ -1,25 +0,0 @@
-name: linkcheck
-
-on:
-  push:
-    branches:
-      - master
-  pull_request:
-
-jobs:
-  linkcheck:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v2
-
-      - name: set up python 3
-        uses: actions/setup-python@v2
-        with:
-          python-version: '3.x'
-          cache: 'pip'
-          
-      - name: install dependencies
-        run: pip install -r requirements.txt
-
-      - name: Verify links
-        run: python linkcheck.py

.github/workflows/linkcheck.yml

@@ -0,0 +1,16 @@
+name: linkcheck
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+
+jobs:
+  linkcheck:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Verify links
+        run: bash ci/docker-linkcheck.sh

ci/docker-linkcheck.sh

@@ -0,0 +1,16 @@
+#! /bin/bash
+
+set -e
+set -o pipefail
+set -x
+
+this_dir="$(readlink -f -- "$(dirname -- "${BASH_SOURCE[0]}")")"
+image="python:3-alpine"
+
+# as we use a pipe to stdin and thus cannot use -it, Ctrl-C does not work by default
+# turns out that combining -i with --init solves that problem
+# see https://stackoverflow.com/a/60812082
+docker run --rm -i -v "$this_dir"/..:/ws -w /ws --init "$image" sh <<\EOF
+    pip install -r requirements.txt
+    python linkcheck.py
+EOF