ci: publish Signet image to GHCR for prod deploy

.github/workflows/build-image.yml

@@ -0,0 +1,32 @@
+name: build-image
+# Build the Signet production image in the cloud and push to GHCR, so the
+# Lightsail box (2 GB, can't build) just pulls. Triggers on main + manual.
+on:
+  push:
+    branches: [main]
+  workflow_dispatch:
+permissions:
+  contents: read
+  packages: write
+concurrency:
+  group: build-signet-${{ github.ref }}
+  cancel-in-progress: true
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: docker/setup-buildx-action@v3
+      - uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: Dockerfile
+          push: true
+          tags: ghcr.io/ministryofmany/ministry-signet:latest
+          cache-from: type=gha
+          cache-to: type=gha,mode=max

Cargo.toml

@@ -5,7 +5,7 @@ edition = "2021"
 # 1.87: usize::is_multiple_of (used in config.rs key-bits validation) was
 # stabilized in 1.87. The codebase already relies on it, so this is the true
 # minimum supported version.
-rust-version = "1.87"
+rust-version = "1.88"
 description = "Hardened partially-blind RSA signing service for FreedInk vote tokens"
 license = "MIT"
 publish = false

Dockerfile

@@ -1,6 +1,6 @@
 # Multi-stage build for the Signet blind-signing service.
 # Stage 1: build a static-ish release binary.
-FROM rust:1.87-slim-bookworm AS builder
+FROM rust:1.88-slim-bookworm AS builder
 
 WORKDIR /build