Add workflow manual trigger (#219)

Allow maintainers to manually trigger a workflow on the main branch to regenerate Docker images. The last workflow on the main branch cannot be manually triggered after the logs retention period has expired.

Author: jngrad

.github/workflows/build.yml

@@ -3,6 +3,7 @@ name: Docker images
 on:
   push:
   pull_request:
+  workflow_dispatch:
 
 jobs:
   images:

.gitignore

@@ -1,3 +1,7 @@
+# ESPResSo Dockerfiles
+
 ![Docker images](https://github.com/espressomd/docker/workflows/Docker%20images/badge.svg)
-# docker
-Docker containers for testing espresso.
+
+This repository manages Dockerfiles and Docker images for the ESPResSo project.
+
+Branch names follow the same naming convention as on the ESPResSo project.