Add make publish: build the image on the Docker host and push it to the registry

script/publish runs script/docker with DOCKER_HOST set to the Docker
host (ssh://root@fsn1app1 unless set) and pushes the image to
git.eeqj.de/sneak/smallwebwaf under four tags: the UTC date as
YYYY-MM-DD, the short commit hash, the branch name and latest. It
refuses a working tree with uncommitted changes, since the hash tag
names a commit.

README.md

@@ -665,6 +665,11 @@ on the host.
   `make check`.
 - `script/docker`: builds the image, whose build runs the tests and the linter
   first. `make docker`.
+- `script/publish`: runs `script/docker` on the Docker host `DOCKER_HOST`
+  (`ssh://root@fsn1app1` unless set) and pushes the image to `PUBLISH_IMAGE`
+  (`git.eeqj.de/sneak/smallwebwaf` unless set) under four tags: the UTC date as
+  `YYYY-MM-DD`, the short commit hash, the branch name and `latest`. Refuses a
+  working tree with uncommitted changes. `make publish`.
 - `script/cibuild`: what CI runs: `script/bootstrap`, `script/check`, then the
   image build.
 - `script/precommit`: run by the git pre-commit hook; runs `script/check`.