We don't have any clear pictures of the bottom of the console or the side of the dock, so this is partially speculation based on the poor pictures that we do have.
I found two pictures that show us the bottom with enough detail to see anything. You can definitely see a charging port in the middle. It's not completely clear whether it's Type-C, microUSB, or a new proprietary connector, but the shape of the edges and connector width makes me lean toward USB Type-C. It's definitely not like the existing 3DS or GamePad chargers and it's clearly not microUSB3 either.
The bottom also has a barcode sticker and what looks like four screw holes around the USB port, but no exposed metal contacts like you might expect of some docks. The back of the console doesn't have any either. I believe this means Switch's dock relies on this USB Type-C port and has a protruding connector like some phone docks you can find today. I'm a little wary of this idea, but USB Type-C is actually rated for an exceptional number of insertion cycles so it'll probably hold up. It looks like the console's back has some shaped plastic below the sticker to help you line it up perfectly.
The choice of USB Type-C makes a lot of sense considering Nintendo's needs of simultaneously using one port for HDMI 1.4 video, USB host (2+ USB-A), and charging. Instead of buying new portable chargers, you could probably (depending on voltage) share the charger with newer Android phones or even some laptops. Depending on the video output format used, there might even be existing laptop docks that will work with Switch.
I can't find any articles about this.