Mask values in a .env file for a screencast
05AB1E, 12 bytes

Uhh, I'd just solve it anyway...

'=¡`g'•иJ'=ý


Try it online!

Explanation

'=¡          Split by equal signs
   `         Put both parts onto the stack
    g        Find the length of the second item
     '•иJ    Repeat that many •'s
         '=ý Join by the equal sign