Modify Dockerfile to allow docker cross build

Provide Readme.armhf with short instructions

Dockerfile

@@ -1,19 +1,21 @@
-FROM nginx:1.17.3
+ARG  DIST=nginx:1.17.3
+FROM $DIST
+ARG  ARCH=amd64
 
 WORKDIR /root
 
 ENV S6_OVERLAY_VERSION v1.22.1.0
 ENV DOCKER_GEN_VERSION 0.7.4
 ENV ACME_TINY_VERSION 4.1.0
 
-ADD https://github.com/just-containers/s6-overlay/releases/download/$S6_OVERLAY_VERSION/s6-overlay-amd64.tar.gz /tmp/
-ADD https://github.com/jwilder/docker-gen/releases/download/$DOCKER_GEN_VERSION/docker-gen-linux-amd64-$DOCKER_GEN_VERSION.tar.gz /tmp/
+ADD https://github.com/just-containers/s6-overlay/releases/download/$S6_OVERLAY_VERSION/s6-overlay-$ARCH.tar.gz /tmp/
+ADD https://github.com/jwilder/docker-gen/releases/download/$DOCKER_GEN_VERSION/docker-gen-linux-$ARCH-$DOCKER_GEN_VERSION.tar.gz /tmp/
 ADD https://raw.githubusercontent.com/diafygi/acme-tiny/$ACME_TINY_VERSION/acme_tiny.py /bin/acme_tiny
 
-RUN tar xzf /tmp/s6-overlay-amd64.tar.gz -C / &&\
-    tar -C /bin -xzf /tmp/docker-gen-linux-amd64-$DOCKER_GEN_VERSION.tar.gz && \
-    rm /tmp/docker-gen-linux-amd64-$DOCKER_GEN_VERSION.tar.gz && \
-    rm /tmp/s6-overlay-amd64.tar.gz && \
+RUN tar xzf /tmp/s6-overlay-$ARCH.tar.gz -C / &&\
+    tar -C /bin -xzf /tmp/docker-gen-linux-${ARCH}-$DOCKER_GEN_VERSION.tar.gz && \
+    rm /tmp/docker-gen-linux-$ARCH-$DOCKER_GEN_VERSION.tar.gz && \
+    rm /tmp/s6-overlay-$ARCH.tar.gz && \
     rm /etc/nginx/conf.d/default.conf && \
     apt-get update && \
     apt-get install -y python ruby cron iproute2 apache2-utils && \

Readme.armhf

@@ -0,0 +1,5 @@
+#to build armhf image from amd64 machine:
+#Install qemu, see https://github.com/multiarch/qemu-user-static
+docker run --rm --privileged multiarch/qemu-user-static --reset -p yes
+#Pass relevant args to the docker build cmd
+docker build  --build-arg DIST=arm32v7/nginx:1.17.3 --build-arg ARCH=armhf -t https-portal:armhf-master .