Add --abs-proxy-base-path for when code-server is not at the root (#6958)

src/node/cli.ts

@@ -53,6 +53,7 @@ export interface UserProvidedCodeArgs {
   "disable-getting-started-override"?: boolean
   "disable-proxy"?: boolean
   "session-socket"?: string
+  "abs-proxy-base-path"?: string
 }
 
 /**
@@ -279,6 +280,10 @@ export const options: Options<Required<UserProvidedArgs>> = {
     short: "w",
     description: "Text to show on login page",
   },
+  "abs-proxy-base-path": {
+    type: "string",
+    description: "The base path to prefix to all absproxy requests",
+  },
 }
 
 export const optionDescriptions = (opts: Partial<Options<Required<UserProvidedArgs>>> = options): string[] => {