Use make to build ameba in docker, add installation info

.dockerignore

@@ -1,8 +1,4 @@
 .*
-Dockerfile
-Makefile
-bench/
-bin/
-spec/
-
-*.html
+!Makefile
+!shard.yml
+!src

Dockerfile

@@ -1,9 +1,9 @@
 FROM alpine:3.8 as builder
-RUN apk add --update crystal shards openssl-dev yaml-dev libxml2-dev musl-dev
+RUN apk add --update crystal shards openssl-dev yaml-dev musl-dev make
 RUN mkdir /ameba
 WORKDIR /ameba
 COPY . /ameba/
-RUN shards build --release
+RUN make clean && make
 
 FROM alpine:3.8
 RUN apk add --update openssl yaml pcre gc libevent libgcc

README.md

@@ -82,6 +82,20 @@ $ brew tap veelenga/tap
 $ brew install ameba
 ```
 
+### Docker
+
+Build the image:
+
+```sh
+$ docker build -t ameba/ameba .
+```
+
+To use the resulting image on a local source folder, mount the current (or target) directory into `/src`:
+
+```sh
+$ docker run -v $(pwd):/src ameba/ameba
+```
+
 ### From sources
 
 ```sh