Add release process

Copied from containernetworking/cni

The only difference is that the release archives also include the the
word "plugin"

scripts/release.sh

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+set -xe
+
+SRC_DIR="${SRC_DIR:-$PWD}"
+BUILDFLAGS="-a --ldflags '-extldflags \"-static\"'"
+
+TAG=$(git describe --tags --dirty)
+RELEASE_DIR=release-${TAG}
+
+OUTPUT_DIR=bin
+
+# Always clean first
+rm -Rf ${SRC_DIR}/${RELEASE_DIR}
+mkdir -p ${SRC_DIR}/${RELEASE_DIR}
+
+docker run -i -v ${SRC_DIR}:/opt/src --rm golang:1.8-alpine \
+/bin/sh -xe -c "\
+    apk --no-cache add bash tar;
+    cd /opt/src; umask 0022;
+    for arch in amd64 arm arm64 ppc64le s390x; do \
+        CGO_ENABLED=0 GOARCH=\$arch ./build.sh ${BUILDFLAGS}; \
+        for format in tgz; do \
+            FILENAME=cni-plugins-\$arch-${TAG}.\$format; \
+            FILEPATH=${RELEASE_DIR}/\$FILENAME; \
+            tar -C ${OUTPUT_DIR} --owner=0 --group=0 -caf \$FILEPATH .; \
+            if [ \"\$arch\" == \"amd64\" ]; then \
+                cp \$FILEPATH ${RELEASE_DIR}/cni-${TAG}.\$format; \
+            fi; \
+        done; \
+    done;
+    cd ${RELEASE_DIR};
+      for f in *.tgz; do sha1sum \$f > \$f.sha1; done;
+      for f in *.tgz; do sha256sum \$f > \$f.sha256; done;
+      for f in *.tgz; do sha512sum \$f > \$f.sha512; done;
+    cd ..
+    chown -R ${UID} ${OUTPUT_DIR} ${RELEASE_DIR}"