Dashboard missing when starting in a Docker container

For some reason, starting `distributed` in a Docker container results in the `bokeh`-based dashboard being unavailable. This is when the correct port (8787) is forwarded from the container. Have also verified that the Bokeh dashboard is running on that port in the container. Strangely the same issue does not occur when using `dask-drmaa` or when using `distributed` outside of a Docker container.

Example:

```python
>>> import distributed
>>> cluster = distributed.LocalCluster()
>>> client = distributed.Client(cluster)
```

Environment:

```yaml
name: test
channels:
- conda-forge
- defaults
dependencies:
- blas=1.1=openblas
- bokeh=0.12.14=py36_1
- ca-certificates=2018.1.18=0
- certifi=2018.1.18=py36_0
- click=6.7=py_1
- cloudpickle=0.5.2=py_0
- cytoolz=0.9.0.1=py36_0
- dask-core=0.17.2=py_0
- distributed=1.21.4=py36_0
- heapdict=1.0.0=py36_0
- jinja2=2.10=py36_0
- markupsafe=1.0=py36_0
- msgpack-python=0.5.6=py36_0
- ncurses=5.9=10
- numpy=1.14.2=py36_blas_openblas_200
- openblas=0.2.20=7
- openssl=1.0.2n=0
- packaging=17.1=py_0
- pip=9.0.1=py36_1
- psutil=5.4.3=py36_0
- pyparsing=2.2.0=py36_0
- python=3.6.5=0
- python-dateutil=2.7.2=py_0
- pyyaml=3.12=py36_1
- readline=7.0=0
- setuptools=39.0.1=py36_0
- six=1.11.0=py36_1
- sortedcontainers=1.5.9=py36_0
- sqlite=3.20.1=2
- tblib=1.3.2=py36_0
- tk=8.6.7=0
- toolz=0.9.0=py_0
- tornado=5.0.1=py36_1
- wheel=0.30.0=py36_2
- xz=5.2.3=0
- yaml=0.1.7=0
- zict=0.1.3=py_0
- zlib=1.2.11=0
- libgfortran=3.0.0=1
```

Using macOS 10.11 with Docker for Mac 18.03.0-ce-mac58 (23607) from the Edge channel. Also used the `condaforge/linux-anvil` image as a base image with digest `sha256:e4dbddbf5c1d1e5143b003608c2a29e6437cdbdb6c4b748cd09fee35f63ab8b3`. Though expect any minimal Docker image with a copy of Miniconda to do the install would also work.