Add a dockerhub build hook to build from different base images

Author: coldfix

README.md

@@ -6,7 +6,7 @@
 GitHub allows developers to run GitHub Actions workflows on your own runners.
 This Docker image allows you to create your own runners on Docker.
 
-For now, there is only a Debian Buster image, but I may add more variants in the future. Feel free to create an issue if you want another base image.
+For now, there are only Debian Buster and Ubuntu Focal images, but I may add more variants in the future. Feel free to create an issue if you want another base image.
 
 ## Important notes
 

docker/hooks/build

@@ -0,0 +1,19 @@
+#! /usr/bin/env bash
+set -ex
+
+# This overrides Docker Hub's build command in order to specify the base image
+# as build argument. For more information on build hooks and environment
+# variables, see: https://docs.docker.com/docker-hub/builds/advanced/
+
+TAG=${IMAGE_NAME##*:}
+if [[ $TAG == latest ]]; then
+    TAG=debian-buster-slim
+fi
+
+# Replace first '-' by ':'
+FROM=${TAG/-/:}
+
+docker build . \
+    --build-arg FROM=$FROM \
+    -f ${DOCKERFILE_PATH:-Dockerfile} \
+    -t $IMAGE_NAME