Update Makefile after migration to Alpine.

The ChirpStack image base has recently changed from Debian to Alpine,
causing the Makefile import-lorawan-devices task to fail.

Fixes #85.

Author: brocaar

Makefile

@@ -1,6 +1,5 @@
 import-lorawan-devices:
-	docker-compose run --rm --entrypoint bash --user root chirpstack -c '\
-		apt-get update && \
-		apt-get install -y make git && \
+	docker-compose run --rm --entrypoint sh --user root chirpstack -c '\
+		apk add --no-cache git && \
 		git clone https://github.com/brocaar/lorawan-devices /tmp/lorawan-devices && \
 		chirpstack -c /etc/chirpstack import-legacy-lorawan-devices-repository -d /tmp/lorawan-devices'