fix(Dockerfile): package.json isn't copied before running yarn install

.dockerignore

@@ -0,0 +1,5 @@
+node_modules
+.idea
+public
+.cache
+.github

Dockerfile

@@ -20,8 +20,8 @@ RUN apk --no-cache add shadow \
     file \
     pkgconf
 WORKDIR /usr/src/app
+COPY package.json yarn.lock ./
 RUN yarn global add gatsby-cli@3.3.0
-COPY yarn.lock ./
 RUN yarn
 COPY . ./
 RUN yarn build