docs: fix image build instructions

Dockerfile

@@ -1,37 +1,17 @@
-# create build from source
-FROM python:3 AS build
-
-WORKDIR /build
-
-RUN set -xe \
-    && apt-get update -y \
-    && apt-get install -y zip pandoc \
-    && curl -L https://github.com/ytdl-org/youtube-dl/archive/master.zip -o youtube-dl-master.zip \
-    && unzip youtube-dl-master.zip
-
-WORKDIR /build/youtube-dl-master
-
-RUN set -xe \
-    && make
-
-# add the build to the web container
 FROM node:12-alpine
 
-WORKDIR /app
+# install youtube-dl
-
-# copy the build
-COPY --from=build /build/youtube-dl-master/youtube-dl /usr/local/bin
-
-# install dependencies
 RUN set -xe \
-    && chmod  a+rx /usr/local/bin/youtube-dl \
     && apk add --no-cache ca-certificates \
                           ffmpeg \
                           openssl \
                           python3 \
-    && ln -s /usr/bin/python3 /usr/bin/python
+    && pip3 install --upgrade pip \
+    && pip3 install youtube-dl
 
 # install app dependencies
+WORKDIR /app
+
 RUN npm install -g yarn
 COPY package.json yarn.lock ./
 RUN yarn install