The alt text you entered is included whenever you export this InDesign file as an EPUB, HTML, or PDF document. However, it is not added to the metadata for the image file itself, so you'll need to re-enter it if you use the image in a different document. To ensure the alt text remains with the image, enter it as metadata in Adobe Bridge.
You can import alt text:
If alt text was entered for an image in Microsoft Word or Adobe Bridge, for example, you can easily assign the same text in InDesign CC.
When you export the document, the alt text you've assigned will travel with the image.
More options for adding Alternative Text to images in InDesign documents are discussed in the InDesign CC Accessibility Overview.