add docker build docs page

docs/wiki/docker-build.md

@@ -0,0 +1,38 @@
+# Building the Elpis Docker image
+
+To build an image locally, use this command.
+```
+docker build --tag IMAGE_NAME .
+```
+
+For example,
+```
+docker build --tag coedl/elpis:latest --tag coedl/elpis:0.96.8 .
+```
+
+After building, push to the hub.
+```
+docker push coedl/elpis:latest
+docker push coedl/elpis:0.96.8
+```
+
+Or push all tags.
+```
+docker image push --all-tags coedl/elpis
+```
+
+
+## Build issues
+
+Docker build issues  may be due to Docker storage being full. This may be indicated by error messages such as:
+```
+E: The repository 'http://archive.ubuntu.com/ubuntu focal-updates InRelease' is not signed.
+W: GPG error: http://archive.ubuntu.com/ubuntu focal-backports InRelease: At least one invalid signature was encountered.
+```
+
+Try cleaning space with these commands, and then rebuild.
+```
+docker image prune
+docker container prune
+docker builder prune
+```