perf(dockerfile): bump the OS to bullseye and optimize the build

 - Bump the Debian OS version from buster-slim to bullseye-slim, the current
   stable.
 - That OS bump speeds up installing numpy, since it can now be installed from
   a wheel instead of being compiled from source.
 - Add `MAKEFLAGS="-j$(nproc)"` in front of `pip install` to use all CPUs
   available in those cases where compilation from source is still required.
 - Reorder some instructions to improve docker cache hits.
 - Save the Dockerfile inside the image, for self-documentation purposes.

Dockerfile

@@ -1,22 +1,23 @@
 # Base Image
-FROM debian:buster-slim
+FROM debian:bullseye-slim
 
 ENV DEBIAN_FRONTEND=noninteractive
 
 # Dependencies that don't change with g2p updates and can be cached
 RUN apt-get update -y && apt-get install -y apt-transport-https
 RUN apt-get install -y libffi-dev openssl libssl-dev python3 python3-pip python3-dev build-essential nano git
 
-RUN mkdir /g2p
-RUN mkdir /g2p/g2p
-COPY g2p /g2p/g2p
+# Get g2p-specific dependencies that can also often be cached
+RUN mkdir -p /g2p/g2p
 COPY requirements /g2p/requirements
 COPY requirements.txt /g2p
 COPY setup.py /g2p
-COPY README.md /g2p
+RUN MAKEFLAGS="-j$(nproc)" pip3 install -r /g2p/requirements.txt
 
-# Get g2p-specific dependencies
-RUN pip3 install -r /g2p/requirements.txt
+# Install g2p itself, last
+COPY g2p /g2p/g2p
+COPY README.md /g2p
+COPY Dockerfile /g2p
 RUN pip3 install -e /g2p
 
 # Comment this out if you just want to install g2p in the container without running the studio.