feat: update Dockerfile to add builder user and improve build environment

docker/Dockerfile.build

@@ -5,10 +5,13 @@ ENV CGO_ENABLED=0
 ENV GOOS=linux
 ENV GOARCH=amd64
 
-RUN apk add --no-cache git
+RUN apk add --no-cache git && \
+    adduser -D -s /bin/sh builder
 
 WORKDIR /build
 
+USER builder
+
 COPY go.mod go.sum ./
 RUN go mod download