Added docker build to Makefile to allow build call via make

Makefile

@@ -26,7 +26,7 @@
 #OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 #OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-.PHONY: test install gofmt
+.PHONY: test install gofmt docker
 
 build:
 	go build ./cmd/invertergui/
@@ -45,3 +45,6 @@ test-race:
 
 vet:
 	go vet ./...
+
+docker:
+	docker build --tag invertergui .