ci: Import ci pipeline from original repo

When this repository was split from the original *dustin/sshca*
repository, the CI pipeline was not imported.  It wouldn't have mattered
if it had been, since it wouldn't have worked, anyway, given the path
changes.

ci/common.sh

@@ -0,0 +1,13 @@
+escape_name() {
+    echo "$1" \
+        | tr A-Z a-z \
+        | sed -e 's/[^a-zA-Z0-9._-]/-/g' -e 's/^[.-]/_/'
+}
+
+REGISTRY_URL=git.pyrocufflink.net
+NAMESPACE=containerimages
+NAME="${JOB_NAME#*/}"
+NAME=$(escape_name "${NAME%/*}")
+TAG=$(escape_name "${BRANCH_NAME}")
+
+IMAGE_NAME="${REGISTRY_URL}/${NAMESPACE}/${NAME}"