Update build

[manics]

• Fixes the Makefile so the standalone image is always built from the matching non-standalone image, e.g. make docker-build should work as expected, previously the standalone image ignored the local one.
• Updates the test script so in future it could be used to push to Travis. At present the OMERO.web Docker release process is completely manual since automated builds don't allow multiple images.
• Adds the django-cors-header fix from Cap version of django-cors-header deployed in IDR IDR/deployment#169

This should make it easier to test OMERO.web Docker related PRs. For example ome/openmicroscopy#5948 (comment) could be tested locally by building the standalone image with a CI artifact:

make VERSION=test REPO=test BUILDARGS="\
    --build-arg OMEGO_ADDITIONAL_ARGS=--ci=https://web-proxy.openmicroscopy.org/west-ci/ \
    --build-arg=OMERO_VERSION=OMERO-build" docker-build

docker run -d --name test-web \
    -e CONFIG_omero_web_server__list='[["eel.openmicroscopy.org", 4064, "eel"]]' \
    -e CONFIG_omero_web_debug=true \
    -p 4080:4080 \
    test/omero-web-standalone:latest

[will-moore]

Seems to build and run OK (no errors). test-web seems to be running but I was expecting http://localhost:4080/ to give me something?

[will-moore]

Ah, sorry - I wasn't on this PR branch.
Working fine.
Used docker run -it --rm --name test-web... to run interactively and http://localhost:4080/webclient/ is working fine, logged into eel.

[joshmoore]

Tested with a docker-compose file that matches the description here that I will open as a separate PR. 👍