docs: add installation instructions for Dockerfile

Thank you for the installation instructions which cover so many operating systems. Container environments are sufficiently non-standard that it can be tricky to work out the best way to install and start gcsfuse.

Some of the elements of the current documentation that gave me trouble in a buster-slim container image include:

* not using sudo
* lsb_release, fuse, curl, gnupg not available

## Proposed Addition

Documentation on working with containers covering:
* Installation of gcsfuse and dependencies
* Starting up gcsfuse in the container entrypoint
* Debian/Ubuntu, CentOS/RHEL, and Alpine are all likely base images.

I ended up adding `curl` and `fuse` to an apt-get install block, then inserting the following:

```
ENV GCSFUSE_VERSION 0.27.0
RUN curl -L -O https://github.com/GoogleCloudPlatform/gcsfuse/releases/download/v${GCSFUSE_VERSION}/gcsfuse_${GCSFUSE_VERSION}_amd64.deb \
&& dpkg --install gcsfuse_${GCSFUSE_VERSION}_amd64.deb \
&& rm -Rf gcsfuse_${GCSFUSE_VERSION}_amd64.deb
```