To be able to install add-ons on openHAB 2.5.12 you also need to add the openhab-addons-2.5.12.kar to /openhab/addons in the container. That is because Bintray is no longer with us. 
(openHAB 2.5.x and the sunset of Bintray). :
To be able to install add-ons on openHAB 2.5.12 you also need to add the openhab-addons-2.5.12.kar to /openhab/addons in the container. That is because Bintray is no longer with us. (openHAB 2.5.x and the sunset of Bintray). :