Update drone.yml to new format

.drone.yml

@@ -1,18 +1,15 @@
-workspace:
-  base: /go
-  path: src/deadbeef.codes/steven/mandelmapper
+kind: pipeline
+name: default
 
-pipeline:
-  build:
-    image: golang:latest
-    commands:
-      - go version
-      - go get
-      - CGO_ENABLED=0 GOOS=linux go build -ldflags '-w'
-  publish:
-    image: plugins/docker
-    repo: deadbeef.codes:5000/mandelmapper
-    registry: deadbeef.codes:5000
-    tags:
-      - latest
-      - ${DRONE_COMMIT}
+steps:
+- name: build
+  image: golang
+  pull: always
+  environment:
+    GOOS: linux
+    GOARCH: amd64
+    CGO_ENABLED: 0
+  commands:
+    - go version
+    - go get
+    - go build -ldflags '-w'