As of Jan 2019, Docker Editor is not available from the maintainer's repository. Until such a time as this is rectified, here are instructions for exporting the project to a local repository for installation.
This should be done in an instance of Eclipse for Committers.
The source can be obtained by cloning the maintainer's github repo.
To import, go to File > Import... > General > Existing Projects into Workspace. Navigate to the repository directory as the root and leave other settings as default. Hit Finish.
Go to File > Export... > Plug-In Development > Deployable features.
In the window, check the project eu.openanalytics.editor.docker. In the Destination tab, select Directory and browse to an output directory. In the Options tab, uncheck Categorize repository. Hit Finish.
In your Eclipse installation, go to Help > Install New Software.... In the window, hit Add... new repository. In the dialog box, hit Local and navigate to the path you specified in Destination in the previous step. Hit Add.
In the Install New Software... window, uncheck Group items by category. You should now see Dockerfile Editor. Check it and continue as usual.