Install docker-cli only for leaner image

As only the docker client is executed in the image and not the docker daemon, only the docker-cli package is needed

Dockerfile

@@ -13,8 +13,7 @@ COPY Gemfile Gemfile.lock kamal.gemspec ./
 COPY lib/kamal/version.rb /kamal/lib/kamal/version.rb
 
 # Install system dependencies
-RUN apk add --no-cache build-base git docker openrc openssh-client-default yaml-dev \
-    && rc-update add docker boot \
+RUN apk add --no-cache build-base git docker-cli openssh-client-default yaml-dev \
     && gem install bundler --version=2.6.5 \
     && bundle install