Dockerfile to build + serve HTML documentation
Dockerfile to build + serve HTML documentation

Summary: A Dockerfile to automate building doxygen docs so online HTML docs are always up to date

Test Plan:
```
# build the docker image
docker build -t abc-docs -f Dockerfile-doxygen .
# run the docker image (nginx static HTML)
docker run --rm -p 4000:80 abc-docs
```

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, schancel, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, schancel, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1303

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