docs(assets): tarball path should be absolute (#15750)

Improve docs to indicate that the path to a tarball should be absolute and not relative.

Fixes #15721 

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-ecr-assets/lib/tarball-asset.ts

@@ -16,7 +16,11 @@ import { Construct as CoreConstruct } from '@aws-cdk/core';
  */
 export interface TarballImageAssetProps {
   /**
-   * Path to the tarball.
+   * Absolute path to the tarball.
+   *
+   * It is recommended to to use the script running directory (e.g. `__dirname`
+   * in Node.js projects or dirname of `__file__` in Python) if your tarball
+   * is located as a resource inside your project.
    */
   readonly tarballFile: string;
 }

packages/@aws-cdk/aws-ecs/lib/container-image.ts

@@ -59,7 +59,8 @@ export abstract class ContainerImage {
    * Use this method if the container image has already been created by another process (e.g. jib)
    * and you want to add it as a container image asset.
    *
-   * @param tarballFile Path to the tarball (relative to the directory).
+   * @param tarballFile Absolute path to the tarball. You can use language-specific idioms (such as `__dirname` in Node.js)
+   *                    to create an absolute path based on the current script running directory.
    */
   public static fromTarball(tarballFile: string): ContainerImage {
     return {