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.

sitesvc/Dockerfile

@@ -1,13 +1,16 @@
+# Context is the repository root; sitesvc depends on the shared module.
 FROM golang:1.26-alpine AS builder
 
-WORKDIR /app
+WORKDIR /src
 
-COPY go.mod go.sum ./
-RUN go mod download
+COPY shared/go.mod shared/go.sum ./shared/
+COPY sitesvc/go.mod sitesvc/go.sum ./sitesvc/
+RUN cd sitesvc && go mod download
 
-COPY . .
+COPY shared/ ./shared/
+COPY sitesvc/ ./sitesvc/
 
-RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o /out/sitesvc ./cmd
+RUN cd sitesvc && CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o /out/sitesvc ./cmd
 
 FROM alpine:3.20 AS runner
 

sitesvc/go.mod

@@ -11,6 +11,7 @@ require (
 
 require (
 	github.com/klauspost/compress v1.17.6 // indirect
+	github.com/mrhid6/vantage/shared v0.0.0
 	github.com/xdg-go/pbkdf2 v1.0.0 // indirect
 	github.com/xdg-go/scram v1.2.0 // indirect
 	github.com/xdg-go/stringprep v1.0.4 // indirect