Remove s390x from container building targets

We have decided to remove s390x container image from our building targets, as TPA's base image does not support it, and it is very unlikely to have any practical use case. Please let us know if you are using s390x, since I have not found any vendor that provide vps with s390x ISA, other than IBM Cloud and their developer resource.

https://meetbot.debian.net/tor-meeting/2025/tor-meeting.2025-07-31-16.00.html

16:03:14 <shelikhoo> the first topic is Remove s390x container image build for snowflake-proxy
16:03:20 <shelikhoo> it is from me
16:03:47 <shelikhoo> s390x is a very rare ISA
16:04:10 <shelikhoo> and it is unlikely to be used by a large user base
16:04:21 <shelikhoo> and is not supported by TPA's base image
16:04:59 <shelikhoo> since
16:05:00 <shelikhoo> https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/merge_requests/597
16:05:43 <shelikhoo> we are using TPA's base image as a source of snowflake-proxy's file system template
16:06:06 <shelikhoo> I think we could remove support for s390x without impacting too much users
16:06:21 <meskio> I support that
16:06:58 <shelikhoo> do we know anyone that are using s390x machines?
16:07:12 <shelikhoo> I thing it quite hard to even find such vps
16:07:14 <meskio> I think we have a s390x machine in our CI, I also added support for it in the obfs4-bridge container without thinking
16:07:24 <meskio> but planning to remove it as part of https://gitlab.torproject.org/tpo/anti-censorship/docker-obfs4-bridge/-/issues/25
16:07:41 <shelikhoo> I think we could test the code on s390x
16:07:57 <meskio> shelikhoo: I don't expect anybody to use it, I would remove it and see if someone complains
16:08:25 <shelikhoo> yes
16:08:30 <shelikhoo> let's remove it for now
16:08:34 <meskio> +1