Adding arm32v7 architecture support.

[ninjarobot]

This adds a Dockerfile for arm32v7 support. It differs from the amd64 build as it removes the rocksdb addin since there are no packages for it on the Debian arm32 images. It also removes the hdt addin due to an issue building that addin.

@JanWielemaker aside from those packages, are there any other tests or concerns you have with running on arm32? I tested loading the remaining addins as well as some basic prolog and it seems to work, but you mentioned there were some edge cases from time to time.

[JanWielemaker]

I don't think the current version has arm32 specific issues. Keri and I are running rather extreme stress tests on amd64 as well as aarch64 (arm 64). This exposed a number of race conditions in atom and clause garbage collection. Depending on progress I expect a new stable and development in the coming week. It might be wise to push this through with the next releases.

[ninjarobot]

@JanWielemaker I see the 7.7.7 update is available. Do you think this is a good candidate for adding an arm32 image?

[JanWielemaker]

Yes, I think 7.7.7 is pretty much ok on ARM. Let us try :)

[ninjarobot]

I updated to 7.7.7 and tested on amd64 and arm32. I'm going to merge and then update the official images.

[JanWielemaker]

Thanks! Fingers crossed :)