dockerfile issues, solutions in here: disable buildkit and run last command in own line

I was struggling getting the dockerfile to run.

here are the steps to make it work:

As you know, nvidia runtime needs to be available at buildtime.
**ADDITIONALLY:** because of a regression on dockers part buildkit needs to be disabled via env variable before running docker build

`DOCKER_BUILDKIT=0`

Additionally, for some reason the row
```
RUN cd decord && mkdir build && cd build && cmake .. -DUSE_CUDA=ON -DCMAKE_BUILD_TYPE=Release && make -j2 && cd ../python && python3 setup.py install
```
fails because it cant find numpy (i think)

just changing that to
```
RUN cd decord && mkdir build && cd build && cmake .. -DUSE_CUDA=ON -DCMAKE_BUILD_TYPE=Release && make -j2

RUN pip install numpy

WORKDIR /decord/python
RUN python3 setup.py install
```
fixed that and makes the docker build not fail.