docker: Refactor Dockerfile process for official images (#32515)

Summary:
## Commit Message:

Refactors Dockerfile to be as parallel as possible with caching and adds a new Makefile to build said Dockerfile.

Also updated the README.md to reflect the changes as well as updated some of the verbage around running our latest Docker images.

Adds the new Dockerfile process to our CircleCI workflows

## How to build:

Building the new images is pretty simple, just requires `docker` > 18.06 since the new build process relies on `buildkit` caching and multi-stage build resolving.

### Development images
For `runtime` images:
```
make -f docker.Makefile runtime-image
```

For `devel` images:
```
make -f docker.Makefile devel-image
```

Builds are tagged as follows:
```bash
docker.io/${docker_user:-whoami}/pytorch:$(git describe --tags)-${image_type}
```

Example:
```
docker.io/seemethere/pytorch:v1.4.0a0-2225-g9eba97b61d-runtime
```

### Official images

Official images are the ones hosted on [`docker.io/pytorch/pytorch`](https://hub.docker.com/r/pytorch/pytorch)

To do official images builds you can simply add set the `BUILD_TYPE` variable to `official` and it will do the correct build without building the local binaries:

Example:
```
make -f docker.Makefile BUILD_TYPE=official runtime-image
```

## How to push:

Pushing is also super simple (And will automatically tag the right thing based off of the git tag):

```
make -f docker.Makefile runtime-push
make -f docker.Makefile devel-push
```
Signed-off-by: Eli Uriegas <eliuriegas@fb.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32515

Differential Revision: D19558619

Pulled By: seemethere

fbshipit-source-id: a06b25cd39ae9890751a60f8f36739ad6ab9ac99

Dockerfile

@@ -0,0 +1,73 @@
+# syntax = docker/dockerfile:experimental
+#
+# NOTE: To build this you will need a docker version > 18.06 with
+#       experimental enabled and DOCKER_BUILDKIT=1
+#
+#       If you do not use buildkit you are not going to have a good time
+#
+#       For reference: 
+#           https://docs.docker.com/develop/develop-images/build_enhancements/
+ARG BASE_IMAGE=ubuntu:18.04
+ARG PYTHON_VERSION=3.7
+
+FROM ${BASE_IMAGE} as dev-base
+RUN --mount=type=cache,id=apt-dev,target=/var/cache/apt \
+    apt-get update && apt-get install -y --no-install-recommends \
+        build-essential \
+        ca-certificates \
+        ccache \
+        cmake \
+        curl \
+        git \
+        libjpeg-dev \
+        libpng-dev && \
+    rm -rf /var/lib/apt/lists/*
+RUN /usr/sbin/update-ccache-symlinks
+RUN mkdir /opt/ccache && ccache --set-config=cache_dir=/opt/ccache
+ENV PATH /opt/conda/bin:$PATH
+
+FROM dev-base as conda
+RUN curl -v -o ~/miniconda.sh -O  https://repo.continuum.io/miniconda/Miniconda3-latest-Linux-x86_64.sh  && \
+    chmod +x ~/miniconda.sh && \
+    ~/miniconda.sh -b -p /opt/conda && \
+    rm ~/miniconda.sh && \
+    /opt/conda/bin/conda install -y python=${PYTHON_VERSION} conda-build pyyaml numpy ipython&& \
+    /opt/conda/bin/conda clean -ya
+
+FROM dev-base as submodule-update
+WORKDIR /opt/pytorch
+COPY . .
+RUN git submodule update --init --recursive
+
+FROM conda as build
+WORKDIR /opt/pytorch
+COPY --from=conda /opt/conda /opt/conda
+COPY --from=submodule-update /opt/pytorch /opt/pytorch
+RUN --mount=type=cache,target=/opt/ccache \
+    TORCH_CUDA_ARCH_LIST="3.5 5.2 6.0 6.1 7.0+PTX" TORCH_NVCC_FLAGS="-Xfatbin -compress-all" \
+    CMAKE_PREFIX_PATH="$(dirname $(which conda))/../" \
+    python setup.py install
+
+FROM conda as conda-installs
+ARG INSTALL_CHANNEL=pytorch-nightly
+RUN /opt/conda/bin/conda install -c "${INSTALL_CHANNEL}" -y pytorch torchvision && \
+    /opt/conda/bin/conda clean -ya
+
+FROM ${BASE_IMAGE} as official
+LABEL com.nvidia.volumes.needed="nvidia_driver"
+RUN --mount=type=cache,id=apt-final,target=/var/cache/apt \
+    apt-get update && apt-get install -y --no-install-recommends \
+        ca-certificates \
+        libjpeg-dev \
+        libpng-dev && \
+    rm -rf /var/lib/apt/lists/*
+COPY --from=conda-installs /opt/conda /opt/conda
+ENV PATH /opt/conda/bin:$PATH
+ENV NVIDIA_VISIBLE_DEVICES all
+ENV NVIDIA_DRIVER_CAPABILITIES compute,utility
+ENV LD_LIBRARY_PATH /usr/local/nvidia/lib:/usr/local/nvidia/lib64
+WORKDIR /workspace
+
+FROM official as dev
+# Should override the already installed version from the official-image stage
+COPY --from=build /opt/conda /opt/conda