replace npm with yarn, like in the official container

Dockerfile

@@ -2,15 +2,16 @@ FROM alpine:3.11 as builder
 
 RUN apk add --no-cache \
     git \
-    npm \
+    yarn \
     make \
     bash \
     python3
 
 # to get around errors from differing stack sizes in musl
 # https://wiki.musl-libc.org/functional-differences-from-glibc.html#Thread_stack_size
-RUN npm config set unsafe-perm true
-RUN npm install -g webpack webpack-cli
+#RUN npm config set unsafe-perm true
+#RUN npm install -g webpack webpack-cli
+RUN yarn global add webpack webpack-cli
 
 ENV WIKIJS_VERSION=2.2.51
 
@@ -19,8 +20,13 @@ ADD nextcloud-auth.patch /build/nextcloud-auth.patch
 
 WORKDIR /build
 RUN git apply nextcloud-auth.patch
-RUN npm install --production
-RUN make build
+#RUN npm install --production
+#RUN make build
+RUN yarn cache clean
+RUN yarn --non-interactive
+RUN yarn build
+RUN rm -rf ./node_modules
+RUN yarn --production --frozen-lockfile --non-interactive
 
 FROM thallian/confd-env:latest