Refactor Dockerfile, Add TODOs

all these changelogs only apply to the Docker image.

Changelog-Added: added verification of GPG keys for the bitcoin and litecoin tarballs.
Changelog-Fixed: fixed compilation on all target architectures; each had their own bugs (poetry, missing packages...).
Changelog-Fixed: fixed cargo cross compilation. it was mistakenly using QEMU before.
Changelog-Fixed: fixed CPU compatibility bug described in issue 8456
Changelog-Changed: improve build time by 8.8x
Changelog-Changed: improve image size by 2.07x

more detailed changelog can be found on the PR: #8429

Author: Claudio Raimondi

.dockerignore

@@ -1,3 +1,3 @@
 Dockerfile
 contrib/docker/Dockerfile.*
-target
+target