nix: separate into multiple overlays and expose them from top-level

nix/pkgs/posh.nix

@@ -0,0 +1,32 @@
+# posh makes use of podman to run an encapsulated shell session
+{ pkgs, ... }:
+
+{ image, pull ? "always", global_args ? "", run_args ? "" }:
+
+
+(pkgs.writeScriptBin "posh" ''
+  #! ${pkgs.bash}/bin/bash
+  source /etc/profile
+
+  test -S "$SSH_AUTH_SOCK" && ssh="-v $SSH_AUTH_SOCK:$SSH_AUTH_SOCK -e SSH_AUTH_SOCK"
+  tty -s && tty="-t" || quiet="-q"
+
+  # define these as variables so we can override them at runtime
+  POSH_IMAGE=${image}
+  POSH_PULL=${pull}
+
+  if [ "$1" == "-c" ]; then
+    # We've most likely been spawned by sshd and are interested in $2 whitch contains the command string
+    shift
+    # TODO parse the beginning of the command for POSH_* overrides
+  fi
+
+  exec ${pkgs.podman}/bin/podman \
+    ${global_args} run --rm -i $tty $ssh -v ~/:/root -w /root --network host --pull=''${POSH_PULL} \
+    ${run_args} ''${POSH_IMAGE} $@
+'')
+.overrideAttrs(attrs: attrs // {
+    passthru = {
+      shellPath = "/bin/posh";
+    };
+})