Reorganize for better layering and make sure to use the right pip

Dockerfile

@@ -1,7 +1,5 @@
 FROM amazonlinux:2018.03
 
-COPY requirements.txt quilt/requirements.txt
-
 # Need to set "ulimit -n" to a small value to stop yum from hanging:
 # https://bugzilla.redhat.com/show_bug.cgi?id=1715254#c1
 RUN ulimit -n 1024 && yum -y update && yum -y install \
@@ -13,11 +11,13 @@ RUN ulimit -n 1024 && yum -y update && yum -y install \
     zip \
     && yum clean all
 
+COPY requirements.txt quilt/requirements.txt
+
 RUN python3 -m pip install pip==18.1
 
 # Requirements copied from lambda Python 3.6, but not in base image
 # (Plus Cython which is a build-time requirement for numpy)
-RUN pip install -r quilt/requirements.txt
+RUN python3 -m pip install -r quilt/requirements.txt
 
 # Make it possible to build numpy:
 # https://github.com/numpy/numpy/issues/14147