Rename Dockerfile.windows.1809.amd64 to Dockerfile.windows.1809

Author: TP Honey

docker/Dockerfile.windows.1809.amd64 renamed to docker/Dockerfile.windows.1809

@@ -10,4 +10,4 @@ LABEL maintainer="Drone.IO Community <[email protected]>" `
   org.label-schema.schema-version="1.0"
 
 ADD release/windows/amd64/drone-slack.exe C:/drone-slack.exe
-ENTRYPOINT [ "C:\\drone-slack.exe" ]
+ENTRYPOINT [ "C:\\drone-slack.exe" ]