plugins: correctly output build version, cosmetic cleanups

Now that libcni has the ability to print a version message, plumb it
through correctly.

While we're at it,
- fix import paths
- run gofmt
- add some more comments to sample
- add container runtime swappability for release

scripts/release.sh

@@ -2,11 +2,12 @@
 set -xe
 
 SRC_DIR="${SRC_DIR:-$PWD}"
+DOCKER="${DOCKER:-docker}"
 
 TAG=$(git describe --tags --dirty)
 RELEASE_DIR=release-${TAG}
 
-BUILDFLAGS="-ldflags '-extldflags -static -X main._buildVersion=${TAG}'"
+BUILDFLAGS="-ldflags '-extldflags -static -X github.com/containernetworking/plugins/pkg/utils/buildversion.BuildVersion=${TAG}'"
 
 OUTPUT_DIR=bin
 
@@ -15,7 +16,7 @@ rm -Rf ${SRC_DIR}/${RELEASE_DIR}
 mkdir -p ${SRC_DIR}/${RELEASE_DIR}
 mkdir -p ${OUTPUT_DIR}
 
-docker run -v ${SRC_DIR}:/go/src/github.com/containernetworking/plugins --rm golang:1.10-alpine \
+$DOCKER run -v ${SRC_DIR}:/go/src/github.com/containernetworking/plugins --rm golang:1.10-alpine \
 /bin/sh -xe -c "\
     apk --no-cache add bash tar;
     cd /go/src/github.com/containernetworking/plugins; umask 0022;