Ceramic tile skirting on plywood wall
I would use mastic this type of glue sticks to plywood quite well, if you have any of the grout mix left fill the gap (you want a small gap even if you use Calking to fill the bottom gap.) I use mastic adhesive on verticle walls like this because it sticks well. It looks like your tiles are not two large and although mastic is not really for wet locations mopping won't get it saturated enough to release the tile. After grouting the tile seal it and it will probably out last you. Regular grout will stop some moisture I have seen tile on sheetrock put up this way in shower surrounds last 25-35 years before enough moisture got through from not maintaing the grout that the sheetrock failed.