Revert "Use buildx functions for faster build (mastodon#20692)"

This reverts commit 92734e3.

I believe this commit is causing build failures as follows:

Sending build context to Docker daemon  68.12MB
Error response from daemon: dockerfile parse error line 69: Unknown flag: link

with Docker versions:

vulpine@xub-22:~/dev/mastodon$ docker version
Client: Docker Engine - Community
 Version:           20.10.21
 API version:       1.41
 Go version:        go1.18.7
 Git commit:        baeda1f
 Built:             Tue Oct 25 18:01:58 2022
 OS/Arch:           linux/amd64
 Context:           default
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          20.10.21
  API version:      1.41 (minimum version 1.12)
  Go version:       go1.18.7
  Git commit:       3056208
  Built:            Tue Oct 25 17:59:49 2022
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.6.9
  GitCommit:        1c90a442489720eec95342e1789ee8a5e1b9536f
 runc:
  Version:          1.1.4
  GitCommit:        v1.1.4-0-g5fd4c4d
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

.github/workflows/build-image.yml

@@ -37,8 +37,7 @@ jobs:
         with:
           context: .
           platforms: linux/amd64,linux/arm64
-          builder: ${{ steps.buildx.outputs.name }}
           push: ${{ github.event_name != 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
-          cache-from: type=gha
-          cache-to: type=gha,mode=max
+          cache-from: type=registry,ref=tootsuite/mastodon:edge
+          cache-to: type=inline

Dockerfile

@@ -1,4 +1,3 @@
-# syntax=docker/dockerfile:1.4
 FROM ubuntu:20.04 as build-dep
 
 # Use bash for the shell
@@ -66,8 +65,8 @@ RUN cd /opt/mastodon && \
 FROM ubuntu:20.04
 
 # Copy over all the langs needed for runtime
-COPY --from=build-dep --link /opt/node /opt/node
-COPY --from=build-dep --link /opt/ruby /opt/ruby
+COPY --from=build-dep /opt/node /opt/node
+COPY --from=build-dep /opt/ruby /opt/ruby
 
 # Add more PATHs to the PATH
 ENV PATH="${PATH}:/opt/ruby/bin:/opt/node/bin:/opt/mastodon/bin"