Support resolv.conf and --dns in --oci mode

[dtrudg]

In the native runtime, an /etc/resolv.conf file is prepared and mounted in to the container by default. If the --dns flag is not provided then the content of this file is the same as /etc/resolv.conf on the host.

The --dns flag allows custom dns servers to be set for the container. If specified, the /etc/resolv.conf that is bound into the container will be overridden to contain only nameserver entries for the specified DNS servers.

The experimental --oci mode currently has no resolv.conf handling, so DNS setup has to been done manually. The --oci mode should implement the same approach as the native runtime.

The implementation in the native runtime can be used for guidance. Note the utility function files.ResolvConf that is available.

singularity/internal/pkg/runtime/engine/singularity/container_linux.go

Line 2209 in 7d8e39c

func (c *container) addResolvConfMount(system *mount.System) error {