Stage 2 of the docker-free Firecracker backend (#348): stop building the
fixed infra image on the launch host. The infra VM's rootfs is host- and
bottle-agnostic (authorized_keys + guest IP ride the kernel cmdline, not the
rootfs), so it's built once off-host and published as a versioned, ready-to-
boot ext4; the launch host downloads + verifies + boots it — no Docker, no
image tooling, just HTTP + gunzip.

- infra_artifact.py: version = content hash of the rootfs inputs (the shipped
  bot_bottle package + the three Dockerfiles + the init), so a launch host
  pulls the artifact matching its code and a content change can't silently
  boot a stale rootfs. Pull + sha256-verify (fail-closed) + gunzip from a
  Gitea generic package; base/owner/token configurable, default this Gitea.
- infra_vm.ensure_built/boot default to the pull path; BOT_BOTTLE_INFRA_BUILD=
  local keeps the docker build-from-source path for iterating on Dockerfiles.
- publish_infra.py: the off-host half — builds the images with Docker, mke2fs
  the rootfs (with buildah slack), gzips, and PUTs it to the generic package.

Rollout note: default=pull means a launch 404s until an artifact is published;
until the Gitea packages endpoint is enabled + an artifact published, use
BOT_BOTTLE_INFRA_BUILD=local. Freeze/migrate's remaining docker use is a
separate PR.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UoEZHDjv84ChoZbozQERhJ

@@ -0,0 +66,4 @@
+    h = hashlib.sha256()
+    h.update(f"format={_ARTIFACT_FORMAT}\n".encode())
+    pkg = _REPO_ROOT / "bot_bottle"
+    for path in sorted(pkg.rglob("*.py")):

@@ -0,0 +127,4 @@
+        if args.force:
+            _delete(gz_url, token)
+            _delete(sha_url, token)
+        _put(gz_url, gz.read_bytes(), token)

@@ -0,0 +54,4 @@
+def local_build_requested() -> bool:
+    """True when the operator opted into the dev Docker-build path instead of
+    pulling the published artifact (`BOT_BOTTLE_INFRA_BUILD=local`)."""
+    return os.environ.get("BOT_BOTTLE_INFRA_BUILD", "").strip().lower() == "local"

@@ -0,0 +54,4 @@
+def local_build_requested() -> bool:
+    """True when the operator opted into the dev Docker-build path instead of
+    pulling the published artifact (`BOT_BOTTLE_INFRA_BUILD=local`)."""
+    return os.environ.get("BOT_BOTTLE_INFRA_BUILD", "").strip().lower() == "local"

@@ -0,0 +54,4 @@
+def local_build_requested() -> bool:
+    """True when the operator opted into the dev Docker-build path instead of
+    pulling the published artifact (`BOT_BOTTLE_INFRA_BUILD=local`)."""
+    return os.environ.get("BOT_BOTTLE_INFRA_BUILD", "").strip().lower() == "local"