add README.md file

containers/docker/README.md

+# Docker support for the NP-TMcode project
+
+This Dockerfile allows to create mainly:
+
+- the np-tmcode image that is used by the gitlab pipeline runner to test compile and execution of requested merges
+- the np-tmcode-run image, that contains only the pre-built executables, python test scripts, compiled documentation, and the minimal runtime to run them
+
+In order for the docker build to correctly set the context, so that it can access the directories containing source codes, docker build must be invoked from this directory as:
+
+   > docker build -f Dockerfile ../.. --target <image name>
+
+where `<image name>` is either `np-tmcode` or `np-tmcode-run`. One may also add the docker build option `-t tagname` to specify the `tagname` the built image should have in his/her registry.
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