sign_ssh_keys: Add hook to sign SSH host cert

The *POST /sshkeys/sign* operation accepts a host name and a list of SSH
host public keys and returns a signed SSH host certificate for each key.
It uses the `step ssh certificate` command to sign the certificates,
which in turn contacts the configured *step-ca* service.  This operation
will allow hosts to obtain their initial certificates.  Once obtained,
the certificates can be renewed directly using the `step ssh renew`
command with the SSH private keys themselves for authentication.

Containerfile

@@ -17,9 +17,12 @@ RUN --mount=type=cache,target=/var/cache/apt \
 RUN --mount=from=build,source=/tmp/build/dist,target=/tmp/wheels \
 	python3 -m pip install -f /tmp/wheels \
 		dch_webhooks \
+		python-multipart \
 		uvicorn \
 	&& :
 
+COPY --from=docker.io/smallstep/step-cli:0.25.0 /usr/local/bin/step /usr/local/bin/step
+
 USER 1000:1000
 
 CMD ["tini", "/usr/local/bin/uvicorn", "dch_webhooks:app"]