ci: add Drone CI

This is my first experiment to make sure I can actually use this in my
CI.

.drone.yml

@@ -0,0 +1,28 @@
+---
+kind: pipeline
+type: exec
+name: Matrix Notifier check
+
+steps:
+- name: Pre-commit checks
+  commands:
+  - nix flake check
+
+- name: Notifiy
+  commands:
+  - nix run .
+  environment:
+    ADDRESS:
+      from_secret: matrix_homeserver
+    ROOM:
+      from_secret: matrix_roomid
+    USER:
+      from_secret: matrix_username
+    PASS:
+      from_secret: matrix_password
+  settings:
+    trigger:
+      status:
+        - failure
+        - success
+...

matrix-notifier

@@ -23,6 +23,17 @@ rawurlencode() {
     echo "$encoded")
 }
 
+default_drone_message() {
+    local msg="Build ${DRONE_BUILD_STATUS}"
+    msg="$msg ${DRONE_REPO_OWNER}/${DRONE_REPO_NAME}/${DRONE_BUILD_NUMBER}"
+    msg="$msg (${DRONE_BRANCH})"
+    printf '%s' "$msg"
+}
+
+if [ "$DRONE" == "true" ] && [ -z "$MESSAGE" ]; then
+    MESSAGE="$(default_drone_message)"
+fi
+
 if [ -z "$USER" ] || [ -z "$PASS" ]; then
     print_err "You must provide USER and PASS"
     exit 1