In Google docs there is no built-in feature to add image caption but you can use some tactics to solve your problem.
3 Ways to add image captions in Google docs
Add image caption in Google docs through Inline
Click an image in which you want to add caption. Now select inline from the below menu as shown in below image and write your captions
Use an Add-ons in Google docs
There is an add-on name as Caption Maker. You can install caption maker and add image captions. This tool helps you to add caption for images, drawings, and tables.
Add image caption from drawings
Go to Menu -> Insert -> Drawing.
Click Image button and Upload your image from computer in Google docs
Click the Textbox button from the menu.
Draw the textbox below your image, write your text there as shown in image.
You can adjust the font size, color, etc.