chore(docker): optimize builds and name images

- Add BuildKit cache mounts for NuGet and npm to persist package
  caches across builds
- Skip redundant restore on dotnet publish with --no-restore
- Add --prefer-offline to npm ci to prefer cached tarballs
- Tag images as randall/backend:latest and randall/frontend:latest
  via compose image: key

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cicd/docker/Dockerfile.backend

@@ -8,12 +8,14 @@ COPY src/backend/src/Randall.Domain/Randall.Domain.csproj                      s
 COPY src/backend/src/Randall.Application/Randall.Application.csproj            src/backend/src/Randall.Application/
 COPY src/backend/src/Randall.Infrastructure/Randall.Infrastructure.csproj      src/backend/src/Randall.Infrastructure/
 COPY src/backend/src/Randall.Api/Randall.Api.csproj                            src/backend/src/Randall.Api/
-RUN dotnet restore "src/backend/src/Randall.Api/Randall.Api.csproj"
+RUN --mount=type=cache,id=nuget,target=/root/.nuget/packages \
+    dotnet restore "src/backend/src/Randall.Api/Randall.Api.csproj"
 
 # Build — copy source only (no bin/obj/db files)
 COPY src/backend/src/ src/backend/src/
-RUN dotnet publish "src/backend/src/Randall.Api/Randall.Api.csproj" \
+RUN --mount=type=cache,id=nuget,target=/root/.nuget/packages \
-    -c Release -o /app/publish
+    dotnet publish "src/backend/src/Randall.Api/Randall.Api.csproj" \
+    -c Release --no-restore -o /app/publish
 
 # Runtime stage
 FROM mcr.microsoft.com/dotnet/aspnet:10.0 AS runtime

cicd/docker/Dockerfile.frontend

@@ -4,7 +4,8 @@ WORKDIR /app
 
 # Install dependencies — copy lockfiles first to cache the npm layer
 COPY src/frontend/package.json src/frontend/package-lock.json ./
-RUN npm ci
+RUN --mount=type=cache,id=npm,target=/root/.npm \
+    npm ci --prefer-offline
 
 # Copy source
 COPY src/frontend/index.html ./

cicd/docker/docker-compose.yml

@@ -1,5 +1,6 @@
 services:
   backend:
+    image: randall/backend:latest
     build:
       context: ../..
       dockerfile: cicd/docker/Dockerfile.backend
@@ -20,6 +21,7 @@ services:
       start_period: 10s
 
   frontend:
+    image: randall/frontend:latest
     build:
       context: ../..
       dockerfile: cicd/docker/Dockerfile.frontend