Docker version of tool https://github.com/maaaaz/webscreenshot
Build
$ docker build -t webscreenshot .
Creates webscreenshot directory on hostOS where you can find all the screenshots.
$ docker run -it -v ~/webscreenshot:/webscreenshot/screenshots webscreenshot
After you run the container, be sure to enter the following commands:
$ export DISPLAY=':99.0'
$ Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &
To change user-agent or resolution if you need so, check the .js file.