Make ENTRYPOINT an absolute path so that it will work with docker run --workdir

Many users will want to run comet -p to generate a dummy params file. Running

    docker run hexabio/comet -p

will generate comet.params.new in the root dir of the container which is inaccessible to the user. If they want to dump the file to the current directory, they would have to run something like

    docker run -v $PWD:/tmp -w /tmp hexabio/comet -p

but this won't run because the entrypoint is constructed as a relative path, and the comet executable does not exist in /tmp.

With an absolute path for ENTRYPOINT, the command above dumps the comet.params.new in the current directory.