Improve dockerization

[kba]

trafficstars

• [x] Ensure ocrd/core is up-to-date and auto-building
• [ ] Test Circle CI with ocrd/core base image
• [ ] Test Circle CI with ocrd/tesserocr base image
• [ ] Test running make test-cli through docker (i.e. instead of ocrd-tesserocr-recognize: docker run ocrd/tesserocr ocrd-tesserocr-recognize)

[bertsky]

Do we really need to run CI on the Docker images? Seems redundant...

(Also, it would require cloning the repo and mounting tests and test data into the container...)