update: readme from v2.0.0 to v2.1.0

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

.dockerignore

@@ -1,5 +1,7 @@
 .git
 .github
+.hadolint.yaml
 LICENSE
 README.md
+action.yml
 images

.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 && \

README.md

@@ -63,7 +63,7 @@ jobs:
         args: --gc --minify --cleanDestinationDir
 
     - name: deploy
-      uses: peaceiris/actions-gh-pages@v2.0.0
+      uses: peaceiris/actions-gh-pages@v2.1.0
       env:
         ACTIONS_DEPLOY_KEY: ${{ secrets.ACTIONS_DEPLOY_KEY }}
         PUBLISH_BRANCH: gh-pages
@@ -78,8 +78,8 @@ You can pull a public docker image from Docker Hub.
 By pulling docker images, you can reduce the overall execution time of your workflow. In addition, `latest` tag is provided.
 
 ```diff
-- uses: peaceiris/actions-gh-pages@v2.0.0
-+ uses: docker://peaceiris/gh-pages:v2.0.0
+- uses: peaceiris/actions-gh-pages@v2.1.0
++ uses: docker://peaceiris/gh-pages:v2.1.0
 ```
 
 - [peaceiris/gh-pages - Docker Hub](https://hub.docker.com/r/peaceiris/gh-pages)
@@ -137,7 +137,7 @@ jobs:
       run: mkdocs build
 
     - name: Deploy to GitHub Pages
-      uses: peaceiris/actions-gh-pages@v2.0.0
+      uses: peaceiris/actions-gh-pages@v2.1.0
       env:
         ACTIONS_DEPLOY_KEY: ${{ secrets.ACTIONS_DEPLOY_KEY }}
         PUBLISH_BRANCH: gh-pages