Resolve error, only trigger build on master

Dockerfile

@@ -1,9 +1,3 @@
-FROM golang:alpine AS build-env
-
-ADD . /src
-RUN apk --no-cache add git gcc musl-dev
-RUN cd /src && go get -d && go build -o godns
-
 FROM alpine
 
 MAINTAINER Tyler Stuyfzand <tyler@tystuyfzand.com>
@@ -15,5 +9,5 @@ ENTRYPOINT ["/sbin/tini", "-g", "--"]
 CMD ["godns"]
 
 COPY etc/godns.example.conf /etc/godns.conf
-COPY --from=build-env /src/godns /usr/local/bin/godns
+COPY /build/godns /usr/local/bin/godns
 RUN chmod +x /usr/local/bin/godns