Fix Docker build for harness workspace dependency on @homelab/db

Switch harness Dockerfile to pnpm with repo root build context so
workspace:^ dependency on @homelab/db resolves. Use .dockercontext
marker file to opt individual apps into root context builds while
keeping web/api on their local app context.

apps/harness/.dockercontext

@@ -0,0 +1 @@
+root

apps/harness/Dockerfile

@@ -1,16 +1,23 @@
 FROM node:20-alpine AS base
+RUN corepack enable && corepack prepare pnpm@latest --activate
 
 FROM base AS deps
 RUN apk add --no-cache libc6-compat
 WORKDIR /app
-COPY package.json ./
-RUN npm install
+
+# Copy workspace root config + relevant package.jsons for install
+COPY pnpm-workspace.yaml pnpm-lock.yaml package.json ./
+COPY packages/db/package.json ./packages/db/package.json
+COPY apps/harness/package.json ./apps/harness/package.json
+
+RUN pnpm install --frozen-lockfile --filter @homelab/harness...
 
 FROM base AS builder
 WORKDIR /app
-COPY --from=deps /app/node_modules ./node_modules
-COPY . .
-RUN npm run build
+COPY --from=deps /app ./
+COPY packages/db ./packages/db
+COPY apps/harness ./apps/harness
+RUN pnpm --filter @homelab/harness build
 
 FROM base AS runner
 WORKDIR /app
@@ -30,11 +37,10 @@ RUN adduser --system --uid 1001 nextjs
 # Workspace directory for git worktrees (ephemeral)
 RUN mkdir -p /data/harness && chown nextjs:nodejs /data/harness
 
-COPY --from=builder /app/public ./public
-COPY --from=builder --chown=nextjs:nodejs /app/.next/standalone ./
-COPY --from=builder --chown=nextjs:nodejs /app/.next/static ./.next/static
+COPY --from=builder /app/apps/harness/public ./public
+COPY --from=builder --chown=nextjs:nodejs /app/apps/harness/.next/standalone ./
+COPY --from=builder --chown=nextjs:nodejs /app/apps/harness/.next/static ./.next/static
 USER nextjs
 EXPOSE 3100
 ENV PORT=3100
 CMD ["node", "server.js"]
-# trigger rebuild 7