Add arm docker images and fix pipeline dependencies

docker/heroku/Dockerfile

@@ -1,4 +1,4 @@
-FROM plugins/docker:latest
+FROM plugins/docker:linux-amd64
 
 ADD release/linux/amd64/drone-heroku /bin/
 ENTRYPOINT ["/usr/local/bin/dockerd-entrypoint.sh", "/bin/drone-heroku"]

docker/heroku/Dockerfile.linux.arm

@@ -0,0 +1,4 @@
+FROM plugins/docker:linux-arm
+
+ADD release/linux/arm/drone-heroku /bin/
+ENTRYPOINT ["/usr/local/bin/dockerd-entrypoint.sh", "/bin/drone-heroku"]