I have been an off road rider for 40 years. A dozer will excel in some areas of track building but honestly, I think you can be better served. For starters, your spending $20K on a relatively small dozer. Also, while I do own a Jinma and like it, I have to say, at this point in time, if you are buying Chinese, you better be handy and willing to occasionally explain to your wife why you are replacing a gauge, cheap wiring or possibly a starter, hydraulic pump etc. on your new machine. Not a slam here, I really like my Jinma, but the chinese are just not up to speed with U.S, Japan, Germany, Italy, Korea or anyone else on their equipment yet.
I also think you are going to quickly find that you wish you had something more versitile than a dozer.
I agree that for the money, you are going to be much better served with a good used 40+ HP tractor w/ end loader and a few impliments. Owning property, you will quickly find many uses for this that you never imagined or thought you would ever need. If you are dead set about wanting something only to build a track with, I would go with a tracked skid steer or a medium sized wheel loader. Something that not only allows you to push dirt but also able to lift, load and ESPECIALLY important, selectively place dirt efficiently and easily. A bucket is more versitile than a straight blade with limited lift and also allows for easier contouring of jump faces, whoops etc.
I have never built any thing on a large scale as far as a track but have tried doing small tracks with just a box blade....basically the same principal as a dozer in that you only have the ability to push the soil where you want it and I can tell you, it is great for clearing ground and pushing up occasional piles but unless the terrain is naturally shaped for a track and only requires a little "dressing" to complete, you quickly find out that building a track with only the immediately surrounding dirt will likely leave you with little more than a good pee wee type track. Something for larger bikes and/or more experienced riders requires massive yardage of dirt and you have to have some way of transporting and manipulating/shaping it. Kind of hard to see or imagine if you don't have any experience in this area but trust me, you will need to be able to have an area that you can dedicate it to excavating for material and then machinery that will allow you to transport it efficiently.
Just my $.02.