Ah, I see why the linux build required 'sudo' (which isn't even installed on debian-slim). It wasn't inside a docker! This 'container' key is what I want

 * https://help.github.com/en/actions/reference/workflow-syntax-for-github-actions#jobsjob_idcontainer

Author: maltfield

.github/workflows/build.yml

@@ -11,9 +11,9 @@ jobs:
 
   linux:
     runs-on: ubuntu-latest
+    container: debian:buster-slim
 
     steps:
-    - uses: docker://debian:buster-slim
     - uses: actions/checkout@v2
 
     - name: Execute script to build linux AppImage

build/linux/buildAppImage.sh

@@ -26,7 +26,7 @@ PYTHON_PATH='/usr/bin/python3.7'
 ###################
 
 # install os-level depends
-sudo apt-get update; sudo apt-get -y install python3.7 python3-pip wget rsync fuse
+apt-get update; apt-get -y install python3.7 python3-pip wget rsync fuse
 
 # setup a virtualenv to isolate our app's python depends
 ${PYTHON_PATH} -m pip install --upgrade --user pip setuptools virtualenv