Support _FILE env vars

Closes https://github.com/fosrl/docs-v2/issues/141

server/lib/getEnvOrYaml.ts

@@ -1,3 +1,37 @@
-export const getEnvOrYaml = (envVar: string) => (valFromYaml: any) => {
-    return process.env[envVar] ?? valFromYaml;
+import fs from "fs";
+
+// Resolves an environment variable, also honoring a `<envVar>_FILE` variant
+// that points to a file whose (trimmed) contents should be used as the
+// value. This is the common convention for consuming Docker/Swarm secrets
+// (e.g. mounted at /run/secrets/...) without putting the raw value in the
+// container's environment.
+export const readEnvOrFile = (envVar: string): string | undefined => {
+    const fileEnvVar = `${envVar}_FILE`;
+    const filePath = process.env[fileEnvVar];
+
+    if (filePath) {
+        if (process.env[envVar]) {
+            throw new Error(
+                `Both ${envVar} and ${fileEnvVar} are set. Please set only one.`
+            );
+        }
+
+        try {
+            return fs.readFileSync(filePath, "utf8").trim();
+        } catch (error) {
+            throw new Error(
+                `Failed to read ${fileEnvVar} (${filePath}): ${
+                    error instanceof Error ? error.message : error
+                }`
+            );
+        }
+    }
+
+    return process.env[envVar];
 };
+
+export const getEnvOrYaml =
+    (envVar: string) =>
+    (valFromYaml: string | undefined): string | undefined => {
+        return readEnvOrFile(envVar) ?? valFromYaml;
+    };