This PR adds `Icinga Kubernetes Web` to the image. Note that there's no
release yet for `Icinga Kubernetes Web`. So for releases of the image,
we have to set `MODE` to `snapshot` right before the module gets cloned.
This commit introduces an explicit build mode in get-mods.sh that determines
how the versions of external modules are selected.
Possible modes are:
- release: chooses latest release for each module
- snapshot: uses a snapshot/development version for each module (if available)
In action.bash, the mode is automatically set based on the GITHUB_REF variable.
For tags starting with "v" (i.e. version tags like v2.9.0) it is release, other
tag names are not supported. For branches it is snapshot.
