clean pkgroot; specify workdir instead of in Dockerfile

DEMO:
```
~/src/europa/src/golang/src/github.com/zenoss/redis-mon
# plu@plu-9: make docker-tgz
...
Step 10 : RUN go get github.com/tools/godep
 ---> Using cache
 ---> 7401f1d5e592
Successfully built 7401f1d5e592
docker run --rm -v `pwd`:/go/src/github.com/zenoss/redis-mon -w /go/src/github.com/zenoss/redis-mon -e DUID=1000 -e DGID=1000 zenoss/redis-mon-build:0.1.1 make tgz
godep go build -ldflags "-X main.Version 0.1.1 -X main.Gitcommit '562fe9d376b2af8a24e1524c49b6ee9a86aeb012' -X main.Gitbranch 'feature/simplify-build' -X main.Date 'Thu Sep  4 17:19:55 UTC 2014' -X main.Buildtag '0'"
chown 1000:1000 redis-mon
mkdir -p /tmp/redis-mon-pkgroot-562fe9d376b2af8a24e1524c49b6ee9a86aeb012/usr/bin
cp -v redis-mon /tmp/redis-mon-pkgroot-562fe9d376b2af8a24e1524c49b6ee9a86aeb012/usr/bin
'redis-mon' -> '/tmp/redis-mon-pkgroot-562fe9d376b2af8a24e1524c49b6ee9a86aeb012/usr/bin/redis-mon'
tar cvfz /tmp/redis-mon-562fe9d376b2af8a24e1524c49b6ee9a86aeb012.tgz -C /tmp/redis-mon-pkgroot-562fe9d376b2af8a24e1524c49b6ee9a86aeb012/usr .
./
./bin/
./bin/redis-mon
chown 1000:1000 /tmp/redis-mon-562fe9d376b2af8a24e1524c49b6ee9a86aeb012.tgz
mv /tmp/redis-mon-562fe9d376b2af8a24e1524c49b6ee9a86aeb012.tgz .

~/src/europa/src/golang/src/github.com/zenoss/redis-mon
# plu@plu-9: tar -tzvf redis-mon-562fe9d376b2af8a24e1524c49b6ee9a86aeb012.tgz
drwxr-xr-x root/root         0 2014-09-04 12:19 ./
drwxr-xr-x root/root         0 2014-09-04 12:19 ./bin/
-rwxr-xr-x root/root   7116752 2014-09-04 12:19 ./bin/redis-mon

~/src/europa/src/golang/src/github.com/zenoss/redis-mon
# plu@plu-9: ls -l
total 8928
drwxrwxr-x 3 plu plu    4096 Sep  4 09:50 Godeps
drwxrwxr-x 2 plu plu    4096 Sep  4 12:02 hack
-rw-rw-r-- 1 plu plu   18026 Sep  4 09:50 LICENSE
-rw-rw-r-- 1 plu plu    2284 Sep  4 09:50 main.go
-rw-rw-r-- 1 plu plu    3638 Sep  4 12:19 makefile
-rw-rw-r-- 1 plu plu     108 Sep  4 09:50 README.md
-rw-rw-r-- 1 plu plu    1778 Sep  4 09:50 redis.go
-rwxr-xr-x 1 plu plu 7116752 Sep  4 12:19 redis-mon
-rw-r--r-- 1 plu plu 1969559 Sep  4 12:19 redis-mon-562fe9d376b2af8a24e1524c49b6ee9a86aeb012.tgz
-rw-rw-r-- 1 plu plu    1875 Sep  4 09:50 stats.go
-rw-rw-r-- 1 plu plu       6 Sep  4 09:50 VERSION

~/src/europa/src/golang/src/github.com/zenoss/redis-mon
# plu@plu-9: make clean
go clean
rm -f *.deb
rm -f *.rpm
rm -f *.tgz
rm -fr /tmp/redis-mon-pkgroot-*

```