How do I build an Arm version of Docker File for Qnap Arm based Server

I have a Docker File for my application and I use Docker Hub to build it. This works fine on a Synology DS218+ Disk Station, which is Intel based.

Qnap supports Docker on both Intel and Arm devices with its Container Station software , I have purchased a TS131P to test this out but it failed with exec format error. Apparently I have to build an Arm version of the image, but how do I do this ?

Can I build the image on the Qnap itself somehow ?

Update

So my base image was openjdk:8-jre-alpine, so I have found on DockerHub an arm32 equivalent of this, https://hub.docker.com/r/arm32v6/openjdk/ so now:

• Created a new BitBucket rep
• Copied over Docker File
• Changed first line of Docker File to FROM arm32v6/openjdk:8-jre-alpine
• Created a new Automated Build on Docker linked to this repo

But the build is now failing on the second line

RUN apk --no-cache add \
       curl \
       tini

with

[91mstandard_init_linux.go:190: exec user process caused "exec format error"

Since I am using arm image I assume that apk should be compiled for arm, or do I need to tell Docker Hub to build on Arm rather than Intel ?

The simple answer is you have to build an arm image on an arm server, so I built in on the Arm nas itself, since this supports Docker, this is what I did

• Ensure ContainerStation running on nas server
• ssh nas server (from PC)
• docker build buildfile docker login
• --enter username username
• --enter password password
• docker images (to get imageId of built image)
• docker tag imageId repoName/imageName:latest
• docker push

and this was enough to make arm32 version available to be installed on arm32 machine.

Currently I have two separate images, one for Intel and one for Arm. I understand that there is a way to combine multiple images into a single super image, but I have not attempted that yet. repoName/imageName:latest