inject .env file on Render builds

Dockerfile

@@ -5,7 +5,10 @@ COPY ui/package.json ui/package-lock.json ./
 RUN npm install
 ENV NODE_ENV=production
 COPY ui/ .
-RUN npm run build
+# TODO: this is a Render-specific build-time env var hack and isn't suitable for
+# a published Dockerfile:
+RUN --mount=type=secret,id=_env,dst=/app/.env \
+  npm exec vite build
 
 # Build the Go binary, including embedded UI files:
 FROM golang:1.22.2-alpine AS build-go

ui/src/vite-env.d.ts

@@ -1,2 +1,10 @@
 /// <reference types="./global-type-overrides.d.ts" />
 /// <reference types="vite/client" />
+
+interface ImportMetaEnv {
+  readonly VITE_RIVER_API_BASE_URL: string;
+}
+
+interface ImportMeta {
+  readonly env: ImportMetaEnv;
+}