v0.2.1: set gid on container creation

Set GID on container creation for restricted fs permissions.

Verification signatures:

% sha512sum bastrd*
8805f00f3449012e3376592554931cd5da0e93dfbe5abb0f3fbb7cb32cc78b755df5044a56f887a57fd760cc57771c1c9db2a8b845ebc81e5b69499684d4474f  bastrd-linux-amd64
50e88eafe86f5ac17ef4e6f881c1a9e84e237524ee73bb5e8d63f3041a3f57e5d36cf076c3f2ea03e263b0baf8e35c49527909bf79c224dc516b510bf79543e9  bastrd-linux-amd64.gz