Improve release scripts:

- Fix script that creates manifest-list based multi-arch-images.
    The images need to be pushed first.
  - Use docker instead of gcloud docker helper
  - Make sure docker pushes are properly 'dry run'
  - Added preparation instruction to the release script.

scripts/build-binary

@@ -5,7 +5,7 @@ set -e
 source ./scripts/test_lib.sh
 
 VER=$1
-REPOSITORY="${REPOSITORY:-https://github.com/etcd-io/etcd}"
+REPOSITORY="${REPOSITORY:-git@github.com:etcd-io/etcd.git}"
 
 
 if [ -z "$1" ]; then
@@ -56,7 +56,7 @@ function package {
 }
 
 function main {
-  local proj=$(echo "${REPOSITORY}" | sed 's|^.*/\([^/]*\)$|\1|g')
+  local proj="etcd"
 
   mkdir -p release
   cd release