Release v2.1.0 (#14)

* change: base docker image from ubuntu:18.04 to alpine:3.10.2
* update: .hadolint.yaml to ignore DL3018

Author: peaceiris

.hadolint.yaml

@@ -1,2 +1,2 @@
 ignored:
-  - DL3008
+  - DL3018

Dockerfile

@@ -1,6 +1,7 @@
-FROM ubuntu:18.04
+FROM alpine:3.10.2
 
-RUN apt-get update && apt-get install -y --no-install-recommends \
+RUN apk add --no-cache \
+    bash \
     git \
     openssh-client \
     ca-certificates && \