build(docker): ship the silo binary and translate the legacy argv command

Dockerfile.goreleaser now copies silo to /usr/bin/silo, defaults to
CMD ["silo"], and labels the image as Silo. MINIO_UPDATE_MINISIGN_PUBKEY is
removed from the image environment: with the updater permanently disabled it
was dead weight, and leaving upstream's verification key in a Silo image
implied a trust relationship that does not exist. The MINIO_* runtime
environment variables, ports, volume and health endpoints are unchanged.

The image keeps shipping mcli with an /usr/bin/mc symlink. That is the client,
not a MinIO-branded alias for the server binary, and the Helm post-install job
and existing container scripts call it by name.

docker-entrypoint.sh translates a legacy first argument: `minio server /data`
becomes `silo server /data`, so an existing `command: minio server ...` in
compose or a Pod spec keeps working across the image swap. The translation is
argv-level only - no file named minio is installed, and an explicitly
overridden `entrypoint: /usr/bin/minio` still fails, which is the honest
outcome since that path genuinely no longer exists.

The entrypoint also fixes an unrelated startup hazard it was already carrying:
when the image runs under an arbitrary UID, HOME points at an unreadable /root
and the server probes its default config directory during initialization. It
now falls back to /tmp when HOME is unset, /root, missing or unwritable.

docker-entrypoint_test.sh pins all of it - empty argv, legacy minio, native
silo, bare flags, and an explicit shell command that must stay explicit. It is
wired into make rebrand-guard and into the go.yml, release.yml and
test-release.yml gates, so removing the shim breaks CI rather than breaking
users. Passing locally.

download-static-curl.sh gains checksum verification for the curl it fetches
into the build stage.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

dockerscripts/docker-entrypoint_test.sh

@@ -0,0 +1,75 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+script_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+work_dir="$(mktemp -d)"
+trap 'rm -rf "${work_dir}"' EXIT
+
+mkdir -p "${work_dir}/bin"
+cat >"${work_dir}/bin/silo" <<'EOF'
+#!/bin/sh
+printf '%s\n' silo "$@" >"${ENTRYPOINT_CAPTURE}"
+if [ -n "${ENTRYPOINT_HOME_CAPTURE:-}" ]; then
+	printf '%s\n' "${HOME:-}" >"${ENTRYPOINT_HOME_CAPTURE}"
+fi
+EOF
+chmod +x "${work_dir}/bin/silo"
+
+cat >"${work_dir}/bin/chroot" <<'EOF'
+#!/bin/sh
+printf '%s\n' "$1" "$2" >"${ENTRYPOINT_CHROOT_CAPTURE}"
+shift 2
+exec "$@"
+EOF
+chmod +x "${work_dir}/bin/chroot"
+
+run_case() {
+	local name="$1"
+	local expected="$2"
+	shift 2
+	local capture="${work_dir}/${name}.actual"
+	local expected_file="${work_dir}/${name}.expected"
+
+	ENTRYPOINT_CAPTURE="${capture}" PATH="${work_dir}/bin:${PATH}" \
+		"${script_dir}/docker-entrypoint.sh" "$@"
+	printf '%s' "${expected}" >"${expected_file}"
+	diff -u "${expected_file}" "${capture}"
+}
+
+run_case default $'silo\n'
+run_case server $'silo\nserver\n/data\n' server /data
+run_case option $'silo\n--version\n' --version
+run_case explicit-silo $'silo\nserver\n/data\n' silo server /data
+run_case legacy-minio $'silo\nserver\n/data\n' minio server /data
+
+shell_capture="${work_dir}/shell.actual"
+# shellcheck disable=SC2016 # ENTRYPOINT_CAPTURE is expanded by the child shell.
+ENTRYPOINT_CAPTURE="${shell_capture}" PATH="${work_dir}/bin:${PATH}" \
+	"${script_dir}/docker-entrypoint.sh" sh -c 'printf "%s\n" shell >"${ENTRYPOINT_CAPTURE}"'
+test "$(cat "${shell_capture}")" = shell
+
+rootless_capture="${work_dir}/rootless.actual"
+rootless_home_capture="${work_dir}/rootless-home.actual"
+rootless_chroot_capture="${work_dir}/rootless-chroot.actual"
+ENTRYPOINT_CAPTURE="${rootless_capture}" \
+	ENTRYPOINT_HOME_CAPTURE="${rootless_home_capture}" \
+	ENTRYPOINT_CHROOT_CAPTURE="${rootless_chroot_capture}" \
+	HOME=/root MINIO_USERNAME=silo-user MINIO_GROUPNAME=silo-group \
+	MINIO_UID=1001 MINIO_GID=1002 PATH="${work_dir}/bin:${PATH}" \
+	"${script_dir}/docker-entrypoint.sh" silo --version
+test "$(cat "${rootless_home_capture}")" = /tmp
+test "$(sed -n '1p' "${rootless_chroot_capture}")" = --userspec=1001:1002
+test "$(sed -n '2p' "${rootless_chroot_capture}")" = /
+test "$(cat "${rootless_capture}")" = $'silo\n--version'
+
+arbitrary_uid_capture="${work_dir}/arbitrary-uid.actual"
+arbitrary_uid_home_capture="${work_dir}/arbitrary-uid-home.actual"
+ENTRYPOINT_CAPTURE="${arbitrary_uid_capture}" \
+	ENTRYPOINT_HOME_CAPTURE="${arbitrary_uid_home_capture}" \
+	HOME="${work_dir}/missing-home" PATH="${work_dir}/bin:${PATH}" \
+	"${script_dir}/docker-entrypoint.sh" silo --version
+test "$(cat "${arbitrary_uid_home_capture}")" = /tmp
+test "$(cat "${arbitrary_uid_capture}")" = $'silo\n--version'
+
+echo "docker entrypoint argv compatibility tests passed"