Dynamic version build using dynamic hash verification

[AbcSxyZ]

Following comments from seed PR dogecoin/dogecoin#2337 (comment).

In the current situation, it's possible to build a single version with each Dockerfile because the hash of Dogecoin Core is hardcoded
within the Dockerfile.

By taking a look at Docker security instruction to build Dockerfile, they are suggesting alternatives to avoid this hardcoded hash.

Need extra investigation, I will share my finding about the topic here to see if an improvement is possible.

[patricklodder]

Would it help if we put a process on this repo that checks version / arch / checksum against contents of gitian.sigs? That should be relatively easy to do with GH Actions.

[xanimo]

I think dynamically verifying from gitian.sigs is a great idea.

[patricklodder]

I have implemented the Gitian verification in the Dockerfile in #6, to demonstrate what that would look like.