How to manage relationship with post-doc supervisor when applying for a tenure-track position one year before post-doc ends? I currently hold a post-doctoral position in the UK, which is 3 years long, and I am in my second year. I am wondering how bad would it look for my employer if I apply for a lectureship post that I just saw announced and caught my interest? They ask for my current employer as a reference, so there is a slight chance they will contact them. 

I am not sure how it would look, considering that I supposedly signed up for a 3 years job, and would be leaving a year earlier. I don't want them to think that I don't like my current place or anything like that. If I do get that other job, it wouldn't be too terrible even if they don't like it. But there is also a great chance that I won't get the job and will stay in my current position for another year.

EDIT: this is my second post-doc position.
In my experience supervising post-docs the goal has been to get the post-doc into a tenure-track position.  I've seen post-docs apply for tenure track positions and get them before the end of our funding for the post-doc, and I've considered that to be a success, even though we had go out and find a replacement for the post-doc.  

I would encourage you to discuss this with your current supervisor.  If they're not supportive then you probably won't get the kind of recommendation you would need, and there might not be any point in applying for the position.  If they are supportive then you can apply with some confidence.  Even if you don't get the job, you'll have some experience with the application (and perhaps interview) process.