refactor: use buildx for multiarch container builds

manifest.tmpl

@@ -1,24 +0,0 @@
-image: internal/drone-cleanup-agents:{{#if build.tag}}{{trimPrefix "v" build.tag}}{{else}}latest{{/if}}
-{{#if build.tags}}
-tags:
-{{#each build.tags}}
-  - {{this}}
-{{/each}}
-{{/if}}
-manifests:
-  - image: internal/drone-cleanup-agents:{{#if build.tag}}{{trimPrefix "v" build.tag}}-{{/if}}amd64
-    platform:
-      architecture: amd64
-      os: linux
-
-  - image: internal/drone-cleanup-agents:{{#if build.tag}}{{trimPrefix "v" build.tag}}-{{/if}}arm64
-    platform:
-      architecture: arm64
-      os: linux
-      variant: v8
-
-  - image: internal/drone-cleanup-agents:{{#if build.tag}}{{trimPrefix "v" build.tag}}-{{/if}}arm
-    platform:
-      architecture: arm
-      os: linux
-      variant: v7