[packaging] make docker aware of PKG_VERSION

When packaging, we can now pass a PKG_VERSION environment variable to
docker to force which version is being packaged.
If this variable is not passed to docker, then the version is the latest
git tag.

Change-Id: I8863d6a101e8d85573d8547bab1a667060648018

packaging/README.md

@@ -36,6 +36,10 @@ inside the synergy-service directory.
       docker run -i -v /path/to/synergy-service:/tmp/synergy-service \
                  synergy-centos7-builder
 
+  You can override the package version that will be set during the packaging
+  process by adding `-e "PKG_VERSION=x.y.z"` to the above command line.
+  Otherwise, the package version will be set to the latest git tag.
+
   This actually mount the synergy-service directory to `/tmp/synergy-service` on
   the guest.
 
@@ -58,6 +62,10 @@ inside the synergy-service directory.
       docker run -i -v /path/to/synergy-service:/tmp/synergy-service \
                  synergy-ubuntu14.04-builder
 
+  You can override the package version that will be set during the packaging
+  process by adding `-e "PKG_VERSION=x.y.z"` to the above command line.
+  Otherwise, the package version will be set to the latest git tag.
+
 - the resulting deb should be in the build directory if successful
 
 

packaging/docker/centos7/build.sh

@@ -5,8 +5,10 @@ RPMBUILD=/home/pkger/rpmbuild
 PKG_DIR=/tmp/synergy-service
 
 function get_version() {
+    if [[ -z $PKG_VERSION ]]; then
         cd $PKG_DIR
         export PKG_VERSION=$(git tag -l "*.*.*" | sort -V | tail -1)
+    fi
 }
 
 function setup() {

packaging/docker/ubuntu-14.04/build.sh

@@ -10,8 +10,10 @@ function copy_source() {
 }
 
 function get_version() {
+    if [[ -z $PKG_VERSION ]]; then
         cd $PKG_DIR
         export PKG_VERSION=$(git tag -l "*.*.*" | sort -V | tail -1)
+    fi
 }
 
 function setup() {