allow less verbose container image build (netdata#5187)

[netdata release candidate]

Author: paulfantom

packaging/docker/Dockerfile

@@ -6,6 +6,7 @@
 ARG ARCH=amd64-v3.8
 FROM multiarch/alpine:${ARCH} as builder
 
+ARG OUTPUT="/dev/stdout"
 # Install prerequisites
 RUN apk --no-cache add alpine-sdk \
                        autoconf \
@@ -33,8 +34,7 @@ WORKDIR /opt/netdata.git
 
 # Install from source
 RUN chmod +x netdata-installer.sh && \
-    sync && sleep 1 && \
-    ./netdata-installer.sh --dont-wait --dont-start-it
+    ./netdata-installer.sh --dont-wait --dont-start-it &>${OUTPUT}
 
 # files to one directory
 RUN mkdir -p /app/usr/sbin/ \

packaging/docker/build.sh

@@ -39,9 +39,11 @@ docker run --rm --privileged multiarch/qemu-user-static:register --reset
 
 # Build images using multi-arch Dockerfile.
 for ARCH in "${ARCHITECTURES[@]}"; do
-     eval docker build --build-arg ARCH="${ARCH}-v3.8" \
-                  --tag "${REPOSITORY}:${VERSION}-${ARCH}" \
-                  --file packaging/docker/Dockerfile ./ ${BG}
+     eval docker build \
+     		--build-arg ARCH="${ARCH}-v3.8" \
+     		--build-arg OUTPUT=/dev/null \
+     		--tag "${REPOSITORY}:${VERSION}-${ARCH}" \
+     		--file packaging/docker/Dockerfile ./ ${BG}
 done
 wait