Add git commit id to docker command comment

Author: mofirouz

build/Dockerfile

@@ -12,7 +12,8 @@
 ## See the License for the specific language governing permissions and
 ## limitations under the License.
 
-# docker build "$PWD" --build-arg commit=d79ccd1 --build-arg version=v2.1.0 -t heroiclabs/nakama:2.1.0
+# docker build "$PWD" --build-arg commit="$(git rev-parse --short HEAD)" --build-arg version=v2.1.1 -t heroiclabs/nakama:2.1.1
+# docker build "$PWD" --build-arg commit="$(git rev-parse --short HEAD)" --build-arg version="v2.1.1-$(git rev-parse --short HEAD)" -t heroiclabs/nakama-prerelease:"2.1.1-$(git rev-parse --short HEAD)"
 
 FROM golang:1.11.1-alpine3.7 as builder
 

build/pluginbuilder/Dockerfile

@@ -12,7 +12,8 @@
 ## See the License for the specific language governing permissions and
 ## limitations under the License.
 
-# docker build "$PWD" --file ./Dockerfile.pluginbuilder --build-arg commit=d79ccd1 --build-arg version=v2.1.0 -t heroiclabs/nakama-pluginbuilder:2.1.0
+# docker build "$PWD" --file ./Dockerfile.pluginbuilder --build-arg commit="$(git rev-parse --short HEAD)" --build-arg version=v2.1.1 -t heroiclabs/nakama-pluginbuilder:2.1.1
+# docker build "$PWD" --file ./Dockerfile.pluginbuilder --build-arg commit="$(git rev-parse --short HEAD)" --build-arg version="v2.1.1-$(git rev-parse --short HEAD)" -t heroiclabs/nakama-prerelease:"2.1.1-$(git rev-parse --short HEAD)"
 
 FROM golang:1.11.1-alpine3.7 as builder