Docker-compose build fail

Hi Aurelion. Awesome book and great talk at AIConf last week...

Just tried building the docker image with compose and the build failed on
```
RUN conda update -n base conda
```
with traceback
```
Step 3/33 : RUN conda update -n base conda
---> Running in e52f65e52a18
Traceback (most recent call last):
File "/opt/conda/lib/python3.6/site-packages/conda/exceptions.py", line 789, in __call__
return func(*args, **kwargs)
File "/opt/conda/lib/python3.6/site-packages/conda/cli/main.py", line 73, in _main
from ..base.context import context
File "/opt/conda/lib/python3.6/site-packages/conda/base/context.py", line 23, in
from ..common.configuration import (Configuration, LoadError, MapParameter, PrimitiveParameter,
File "/opt/conda/lib/python3.6/site-packages/conda/common/configuration.py", line 34, in
from .serialize import yaml_load
File "/opt/conda/lib/python3.6/site-packages/conda/common/serialize.py", line 28, in
yaml = get_yaml()
File "/opt/conda/lib/python3.6/site-packages/conda/_vendor/auxlib/decorators.py", line 56, in _memoized_func
result = func(*args, **kwargs)
File "/opt/conda/lib/python3.6/site-packages/conda/common/serialize.py", line 17, in get_yaml
import ruamel_yaml as yaml
ValueError: source code string cannot contain null bytes

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/opt/conda/bin/conda", line 11, in
sys.exit(main())
File "/opt/conda/lib/python3.6/site-packages/conda/cli/main.py", line 126, in main
return conda_exception_handler(_main, *args)
File "/opt/conda/lib/python3.6/site-packages/conda/exceptions.py", line 1025, in conda_exception_handler
return_value = exception_handler(func, *args, **kwargs)
File "/opt/conda/lib/python3.6/site-packages/conda/exceptions.py", line 792, in __call__
return self.handle_exception(exc_val, exc_tb)
File "/opt/conda/lib/python3.6/site-packages/conda/exceptions.py", line 832, in handle_exception
return self.handle_unexpected_exception(exc_val, exc_tb)
File "/opt/conda/lib/python3.6/site-packages/conda/exceptions.py", line 844, in handle_unexpected_exception
self.print_error_report(error_report)
File "/opt/conda/lib/python3.6/site-packages/conda/exceptions.py", line 882, in print_error_report
from .base.context import context
File "/opt/conda/lib/python3.6/site-packages/conda/base/context.py", line 23, in
from ..common.configuration import (Configuration, LoadError, MapParameter, PrimitiveParameter,
File "/opt/conda/lib/python3.6/site-packages/conda/common/configuration.py", line 34, in
from .serialize import yaml_load
File "/opt/conda/lib/python3.6/site-packages/conda/common/serialize.py", line 28, in
yaml = get_yaml()
File "/opt/conda/lib/python3.6/site-packages/conda/_vendor/auxlib/decorators.py", line 56, in _memoized_func
result = func(*args, **kwargs)
File "/opt/conda/lib/python3.6/site-packages/conda/common/serialize.py", line 17, in get_yaml
import ruamel_yaml as yaml
ValueError: source code string cannot contain null bytes
ERROR: Service 'handson-ml' failed to build: The command '/bin/sh -c conda update -n base conda' returned a non-zero code: 1
```

Perhaps an issue in a new version of a package being installed?