add mcli/mc from pgsty/mc to Docker image

Rework Dockerfile.goreleaser to download the latest mcli binary from
pgsty/mc GitHub releases, verify its SHA-256 checksum, and install both
mcli and mc (symlink) into the final image alongside minio and curl.
Also add download-static-curl.sh to goreleaser extra_files and enable
workflow_dispatch for the release workflow.

.github/workflows/release.yml

@@ -4,6 +4,11 @@ on:
   push:
     tags:
       - "RELEASE.*"
+  workflow_dispatch:
+    inputs:
+      tag:
+        description: "Release tag (e.g. RELEASE.2026-03-24T12-00-00Z)"
+        required: true
 
 permissions:
   contents: write
@@ -27,7 +32,7 @@ jobs:
       - name: Compute release variables
         run: |
           set -euo pipefail
-          TAG="${GITHUB_REF_NAME}"
+          TAG="${{ github.event.inputs.tag || github.ref_name }}"
           VERSION_HYPHEN="${TAG#RELEASE.}"
           PKG_VERSION="$(echo "${VERSION_HYPHEN}" | sed -E 's/^([0-9]{4})-([0-9]{2})-([0-9]{2})T([0-9]{2})-([0-9]{2})-([0-9]{2})Z$/\1\2\3\4\5\6.0.0/')"
           if [ "${PKG_VERSION}" = "${VERSION_HYPHEN}" ]; then