chore: Remove cruft and simplify Dockerfile (#6633)

Also, change to new syntax of pip for install prefix
Ref: #6543 

pypa/pip#7240 (comment)

Dockerfile

@@ -1,5 +1,4 @@
-FROM python:3.7.4-alpine as base
-LABEL maintainer="Niranjan Rajendran <[email protected]>"
+FROM python:3.7-alpine as base
 
 ####
 
@@ -9,25 +8,19 @@ WORKDIR /install
 
 RUN apk update && \
   apk add --virtual build-deps git gcc python3-dev musl-dev jpeg-dev zlib-dev libevent-dev file-dev libffi-dev openssl && \
-  apk add postgresql-dev && \
-  pip install setuptools
+  apk add postgresql-dev
 
 ADD requirements.txt /requirements.txt
 ADD requirements /requirements/
 
-RUN wget https://bootstrap.pypa.io/ez_setup.py && python ez_setup.py
-
-ENV PYTHONPATH /install/lib/python3.7/site-packages
-RUN pip install --install-option="--prefix=/install" setuptools && \
-    LIBRARY_PATH=/lib:/usr/lib pip install --install-option="--prefix=/install" -r /requirements.txt
+RUN pip install --prefix=/install --no-warn-script-location -r /requirements.txt
 
 ####
 
 FROM base
 
 COPY --from=builder /install /usr/local
-RUN apk --no-cache add postgresql-dev ca-certificates libxslt jpeg zlib file libxml2 git && \
-    pip install git+https://github.com/fossasia/[email protected]#egg=flask-rest-jsonapi
+RUN apk --no-cache add postgresql-libs ca-certificates libxslt jpeg zlib file libxml2
 
 WORKDIR /data/app
 ADD . .

requirements/common.txt

@@ -46,7 +46,7 @@ pytz
 diff-match-patch
 blinker~=1.4
 envparse~=0.2
--e git+https://github.com/fossasia/[email protected]#egg=flask-rest-jsonapi
+git+https://github.com/fossasia/[email protected]#egg=flask-rest-jsonapi
 wtforms~=2.2
 flask-admin~=1.5
 google-compute-engine~=2.8