Use Nim 2.2.6 for Docker builds, unify arm64

nimlang/nim's alpine-regular images cap at 2.2.6 and are now
multi-arch, while Alpine's apk nim is stuck at the segfaulting
2.2.0. Base both arches on 2.2.6-alpine-regular, drop the
separate Dockerfile.arm64, and build ./Dockerfile in the arm64
CI job.

Fixes #1404

.github/workflows/build-docker.yml

@@ -55,7 +55,7 @@ jobs:
         uses: docker/build-push-action@v3
         with:
           context: .
-          file: ./Dockerfile.arm64
+          file: ./Dockerfile
           platforms: linux/arm64
           push: true
           tags: zedeus/nitter:latest-arm64,zedeus/nitter:${{ github.sha }}-arm64