arm processors support

[korney4eg]

Hey umputun, thank you for making rss2twitter. It works nicely.

It seems like those small kind of applications are perfectly match to raspberry pi and others. So would be nice to add docker image for those architectures.

What do you think?

[korney4eg]

It seems that it's not so hard to do. Here is short example.

[umputun]

I don't have pi handy and can't test it however if you would like to make an additional dockerfile for this, feel free to submit a PR.

From what I understand, this should be relatively easy:

1. Add a new dockerfile, i.e.Dockerfile.armhf
2. In the build stage change the target platform to arm, probably GOOS=linux GOARCH=arm GOARM=6
3. For the second stage change base image to alpine:arm32v6
4. Add "app" user, like baseimage does

I think this will be it and the container may work on pi.