Revert "remove drone (replaced by github ci)"

This reverts commit d717ee155e.

.drone.yml

@@ -0,0 +1,37 @@
+---
+kind: pipeline
+name: nixos-tests
+
+steps:
+- name: test
+  image: busybox
+  environment:
+    NIX_REMOTE: daemon
+    PATH: /nix/var/nix/profiles/system/sw/bin/
+    PAGER: cat
+    USER: root
+  volumes:
+    - name: nixstore
+      path: /nix
+    - name: nixconf
+      path: /etc/nix
+    - name: sslcerts
+      path: /etc/ssl
+  commands:
+  - nix-build -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixpkgs-unstable.tar.gz -A sops-install-secrets.tests
+  - nix-build -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixos-20.09.tar.gz -A sops-install-secrets.tests
+volumes:
+- name: nixstore
+  host:
+    path: /nix
+- name: nixconf
+  host:
+    path: /nix/var/nix/profiles/system/etc/nix
+- name: sslcerts
+  host:
+    path: /nix/var/nix/profiles/system/etc/ssl
+---
+kind: signature
+hmac: 5bb657faca0ecd18576534f7e486511d6f698595cc244d9617e660ef2ddb678d
+
+...