Use variables to allow overriding build tools

In particular, this is useful for a situation where we want to use
podman instead of docker, or where adb is not in $PATH.

Makefile

@@ -21,6 +21,11 @@ ANDROIDNDKVER := 21.4.7075529
 
 SDK := ${ANDROID_HOME}/android-sdk-$(PLATFORM)
 
+ADB := adb
+DOCKER := docker
+P4A := p4a
+PYTHON_FOR_ANDROID := python-for-android
+
 # This checks if an environment variable with a specific name
 # exists. If it doesn't, it prints an error message and exits.
 # For example to check for the presence of the ANDROIDSDK environment
@@ -41,9 +46,9 @@ clean:
 	- rm -rf dist/*.apk src/kolibri tmpenv
 
 deepclean: clean
-	python-for-android clean_dists
+	$(PYTHON_FOR_ANDROID) clean_dists
 	rm -r dist || true
-	yes y | docker system prune -a || true
+	yes y | $(DOCKER) system prune -a || true
 	rm build_docker 2> /dev/null
 
 .PHONY: clean-whl
@@ -68,7 +73,7 @@ src/kolibri: clean
 
 .PHONY: p4a_android_distro
 p4a_android_distro: needs-android-dirs
-	p4a create --arch=$(P4A_ARCH)
+	$(P4A) create --arch=$(P4A_ARCH)
 
 .PHONY: needs-version
 needs-version:
@@ -103,19 +108,19 @@ kolibri.apk.unsigned: p4a_android_distro src/kolibri needs-version
 # Makes dummy file
 .PHONY: build_docker
 build_docker: Dockerfile
-	docker build -t android_kolibri .
+	$(DOCKER) build -t android_kolibri .
 
 # Run the docker image.
 # TODO Would be better to just specify the file here?
 run_docker: build_docker
-	./scripts/rundocker.sh
+	env DOCKER="$(DOCKER)" ./scripts/rundocker.sh
 
 install:
-	adb uninstall org.learningequality.Kolibri || true 2> /dev/null
-	adb install dist/*$(ARM_VER)-debug-*.apk
+	$(ADB) uninstall org.learningequality.Kolibri || true 2> /dev/null
+	$(ADB) install dist/*$(ARM_VER)-debug-*.apk
 
 logcat:
-	adb logcat | grep -i -E "python|kolibr| `adb shell ps | grep ' org.learningequality.Kolibri$$' | tr -s [:space:] ' ' | cut -d' ' -f2` " | grep -E -v "WifiTrafficPoller|localhost:5000|NetworkManagementSocketTagger|No jobs to start"
+	$(ADB) logcat | grep -i -E "python|kolibr| `$(ADB) shell ps | grep ' org.learningequality.Kolibri$$' | tr -s [:space:] ' ' | cut -d' ' -f2` " | grep -E -v "WifiTrafficPoller|localhost:5000|NetworkManagementSocketTagger|No jobs to start"
 
 $(SDK)/cmdline-tools:
 	@echo "Downloading Android SDK build tools"

scripts/rundocker.sh

@@ -3,6 +3,7 @@
 set -e
 
 CONTAINER_HOME=/home/kivy
+DOCKER=${DOCKER:-"docker"}
 
 # Specifies the name of the docker volume used to store p4a cache
 P4A_CACHE=p4a_cached_dir_$ARCH
@@ -11,7 +12,7 @@ CID_FILE=kolibri-android-app-container-id.cid.txt
 
 # create the container to be used throughout the script
 # creates a volume for reuse between builds, holding p4a's android distro
-docker create -it \
+${DOCKER} create -it \
   --mount type=volume,src=${P4A_CACHE},dst=${CONTAINER_HOME}/.local \
   --env BUILDKITE_BUILD_NUMBER \
   --env P4A_RELEASE_KEYSTORE=${CONTAINER_HOME}/$(basename "${P4A_RELEASE_KEYSTORE}") \
@@ -35,18 +36,18 @@ if [ "${P4A_RELEASE_KEYSTORE}" ]; then
   if [ -a ${P4A_RELEASE_KEYSTORE} ]; then
     echo -e "Copying the signing key \n\t From ${P4A_RELEASE_KEYSTORE} to ${CONTAINER_ID}:${CONTAINER_HOME}"
     # copy keystore to same location on the container
-    docker cp ${P4A_RELEASE_KEYSTORE} ${CONTAINER_ID}:${CONTAINER_HOME}
+    ${DOCKER} cp ${P4A_RELEASE_KEYSTORE} ${CONTAINER_ID}:${CONTAINER_HOME}
   fi
 fi
 
 # run the container, generating the apk
 echo "Starting ${CONTAINER_ID}"
-docker start -i ${CONTAINER_ID}
+${DOCKER} start -i ${CONTAINER_ID}
 
 # copy the apk to our host. Handles permissions.
 echo -e "Copying APK \n\t From ${CONTAINER_ID}:${CONTAINER_HOME}/dist/ to ${PWD}"
-docker cp ${CONTAINER_ID}:${CONTAINER_HOME}/dist/ .
+${DOCKER} cp ${CONTAINER_ID}:${CONTAINER_HOME}/dist/ .
 
 # manually remove the container afterward
 echo -n "Removing "
-docker rm ${CONTAINER_ID}
+${DOCKER} rm ${CONTAINER_ID}