[docker] fix broken cjson

mpx/lua-cjson#56

Dockerfile

@@ -1,6 +1,6 @@
 FROM alpine:3.7
 
-LABEL maintainer "Michal Cichra <michal@cichra.cz>"
+LABEL maintainer="Michal Cichra <michal@cichra.cz>"
 ENV LUA_VERSION=5.3 LUACHECK_VERSION=0.21.2
 
 WORKDIR /tmp
@@ -10,10 +10,11 @@ RUN adduser -D -H -h /code -u 9000 -s /bin/false app \
  && apk add --no-cache --virtual build-dependencies \
                        lua${LUA_VERSION}-dev build-base curl ruby-dev icu-dev zlib-dev openssl-dev cmake \
  && luarocks-${LUA_VERSION} install luacheck ${LUACHECK_VERSION} \
- && luarocks-${LUA_VERSION} install lua-cjson \
+ && luarocks-${LUA_VERSION} install lua-cjson 2.1.0-1 \
  && BUNDLE_SILENCE_ROOT_WARNING=1 bundle install --system \
  && apk del build-dependencies \
- && ln -s $(which lua${LUA_VERSION}) /usr/local/bin/lua
+ && ln -s $(which lua${LUA_VERSION}) /usr/local/bin/lua \
+ && lua -e 'require("cjson").encode({})'
 
 USER app
 VOLUME /code