build: build Go images from the repo root for the shared module

Also pins the explicit shared require in both go.mod files. `go mod tidy`
in workspace mode drops it, which breaks the Docker build where no
workspace exists.

server/Dockerfile

@@ -1,17 +1,22 @@
 # Build stage
+#
+# Context is the repository root, not server/, because server depends on the
+# shared module through a replace directive.
 FROM golang:1.26 AS builder
 
-WORKDIR /app
+WORKDIR /src
 
-# Download dependencies first (layer cache)
-COPY go.mod go.sum ./
-RUN go mod download
+# Manifests first so the dependency layer caches independently of source edits.
+COPY shared/go.mod shared/go.sum ./shared/
+COPY server/go.mod server/go.sum ./server/
+RUN cd server && go mod download
 
-# Copy source and build
-COPY . .
+COPY shared/ ./shared/
+COPY server/ ./server/
 
 ARG VERSION=dev
-RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w -X main.Version=${VERSION}" -o /vantage-server ./cmd
+RUN cd server && CGO_ENABLED=0 GOOS=linux go build \
+      -ldflags="-s -w -X main.Version=${VERSION}" -o /vantage-server ./cmd
 
 # Runtime stage
 FROM scratch