add a new chapter

docs/README.md

@@ -94,5 +94,6 @@ might turn out useful.
 
 ## Contents
 
+* [`Container`](container.md) structure
 * [`Tool`](./tool.md) specification
 * [`Input (Parameters and Data)`](./input.md) specification

docs/container.md

@@ -0,0 +1,81 @@
+# Container structure
+
+## tl;dr;
+In order to be recognized as a tool following this specification, a container 
+image has to meet these requirements:
+
+1. A description of the tool, its arguments, parameters and input data has to be present in YAML format at the container location `/src/tool.yml`
+2. The executable script/program has to either store results at the location `/out/` of the container, or print them to the containers StdOut.
+3. A tool execution may be parameterized. The parameterization is stored at `/in/input.json` of the container. Additional input data may be added to `/in/input.json`.
+
+## Description
+
+In order to work properly, there is a minimum required structure, that the 
+conatainer has to follow. In  order to build a container image accordingly,
+there are a number of template repositories on Github:
+
+* [Python template](https://github.com/VForWaTer/tool_template_python)
+* [R template](https://github.com/VForWaTer/tool_template_r)
+* [Node.js template](https://github.com/VForWaTer/tool_template_node)
+* [Octave template](https://github.com/VForWaTer/tool_template_octave)
+* [MATLAB template](https://github.com/VForWaTer/tool_template_matlab)
+
+The core idea is to enforce a specific structure in the container, so that 
+others know where to find things:
+
+```
+/
+|- in/
+|  |- input.json
+|- out/
+|  |- ...
+|- src/
+|  |- tool.yml
+|  |- run.[py/R/js/m]
+|  |- CITATION.cff
+```
+
+Here, the two locations `/in` and `/out` are mount points to pass data and 
+parameters in and out of the container.
+
+### Input parameterization
+
+`/in/input.json` contains the the parameterization for running the tool. 
+There are client libraries for each of the supported languages, which read and 
+validate the parameterization. Runtime CLI parameters are not supported yet, 
+but planned for the future.
+
+### Metadata
+
+`/src/tool.yml` is a mandatory metadata file containing the most important 
+information about the tool.
+
+### Entrypoint
+
+`/src/run.[py/R/js/m]` is the single entrypoint into the tool. 
+Any executable file is supported. Please note, that the tool-specs only support 
+transformation-like tools, that *end* (no service tools).
+
+Please note: In order to work properly with external tools, you implement the 
+execution of the script, as shown in the sample repositories, as the default
+Docker command, **not** entrypoint:
+
+```Dockerfile
+CMD ["python", "run.py"]
+```
+
+This way, you can `exec` or `run` into the container and `cat /src/tool.yml`
+to build frameworks that can read the metadata of each tool available on the system.
+
+### Citation information
+
+Each of the template repositories include a `CITATION.cff` to distribute 
+citation information to users of the tool. You, as a developer, should **replace**
+the citation information in the template repositories. The templates are licensed
+as CC zero.
+
+The `CITATION.cff` file format is described [here](https://citation-file-format.github.io/).
+There is also an online tool to build these files: https://citation-file-format.github.io/cff-initializer-javascript/
+
+CFF files are well supported by Github, Zenodo or Zotero.
+

docs/tool.md

@@ -2,24 +2,8 @@
 
 ## Description
 
-We define a `Tool` to be an executable script/program inside a docker container. In order to be recognized as a `Tool`, the container has to meet a set of requirements:
+We define a `Tool` to be an executable script/program inside a docker container. In order to be recognized as a `Tool`, the container has to meet a set of requirements, which are described [here](./container.md)
 
-1. A description of the tool, its arguments, parameters and input data has to be present in YAML format at the container location `/src/tool.yml`
-2. The executable script/program has to either store results at the location `/out/` of the container, or print them to the containers StdOut.
-3. A tool execution may be parameterized. The parameterization is stored at `/in/input.json` of the container. Additional input data may be added to `/in/input.json`.
-
-Essentially, the following folder and file structure must exist within the container:
-
-```
-/
-|- in/
-|  |- input.json
-|- out/
-|  |- ...
-|- src/
-|  |- tool.yml
-|  |- run.py/.R/.m/.js
-```
 
 ## File specification