optimize Dockefile (#409)

Dockerfile

@@ -1,10 +1,9 @@
 FROM golang:alpine
 EXPOSE 8080 1935
-RUN mkdir /app 
-ADD . /app
 WORKDIR /app
-RUN apk add --no-cache ffmpeg ffmpeg-libs
-RUN apk update && apk add --no-cache gcc build-base linux-headers
-RUN CGO_ENABLED=1 GOOS=linux go build -a -installsuffix cgo -ldflags '-extldflags "-static"' -o owncast .
-WORKDIR /app
-CMD ["/app/owncast"]
+ADD . .
+RUN set -ex && \
+    apk add --no-cache ffmpeg ffmpeg-libs && \
+    apk add --no-cache gcc build-base linux-headers && \ 
+    CGO_ENABLED=1 GOOS=linux go build -a -installsuffix cgo -ldflags '-extldflags "-static"' -o owncast .
+CMD ["/app/owncast"]