Add environment variable option to skip docker build

Signed-off-by: Nathan LeClaire <nathan.leclaire@gmail.com>

script/build

@@ -13,6 +13,11 @@ else
     OS_ARCH_ARG=($2)
 fi
 
-docker build -t docker-machine .
+# Build Docker image unless we opt out of it
+if [[ -z "$SKIP_BUILD" ]]; then
+    docker build -t docker-machine .
+fi
+
+# Get rid of existing binaries
 rm -f docker-machine*
 docker run --rm -v `pwd`:/go/src/github.com/docker/machine docker-machine gox "${OS_PLATFORM_ARG[@]}" "${OS_ARCH_ARG[@]}" -output="docker-machine_{{.OS}}-{{.Arch}}" -ldflags="-w -X github.com/docker/machine/version.GITCOMMIT `git rev-parse --short HEAD`"