Make rake:configure docker friendly.

Commit "Improve build system" tried to make `rake:configure` docker friendly by
attempting to read configuration values from `ENV` first. Turned out that the
only way to set environment variable during build is to use the `ENV` directive
in Dockerfile.

`rake:configure` now looks for an `env.yml` file in the current directory and
exports tuples from it to ENV first.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>
Bind to 0.0.0.0 instead of localhost.

Binding to 0.0.0.0 is docker friendly.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>
gitignore .rake/

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>