Let deploy tools' Dockerfile copies into the build

Arcane adds .arcane.external.Dockerfile to the build context, which the
allowlist in .dockerignore was dropping.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.dockerignore

@@ -2,6 +2,9 @@
 # (data, backups, .env, files a deploy tool adds) stays out of the build,
 # so it can't bloat the upload or change while it's being sent.
 *
+# Build tools may add their own copy of the Dockerfile (Arcane writes
+# .arcane.external.Dockerfile), which has to reach the builder.
+!*Dockerfile*
 !go.mod
 !go.sum
 !cmd/