Dockerfile READMEs added

docker/README_debian.md

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+# Docker GRASS GIS (Debian Linux)
+
+Dockerfile with an [Debian Linux](https://www.debian.org/) image with [GRASS GIS](https://grass.osgeo.org/), [PDAL](https://pdal.io) support and [grass-session](https://github.com/zarch/grass-session/).
+
+Download size of this image is of approximately 2.6 GB.
+
+Build the docker with:
+
+```bash
+$ sudo docker build --build-arg GRASS_VERSION=7.8 \
+                    --build-arg PYTHON_VERSION=3  \
+                    --build-arg PROJ_VERSION=5.2.0 \
+                    --build-arg PROJ_DATUMGRID_VERSION=1.8 \
+                    --file Dockerfile_debian_pdal .
+```
+
+View the images available using `sudo docker images` and open a bash terminal with:
+
+```bash
+$ sudo docker run -i -t 15550df91610 /bin/bash
+bash-5.0#
+```

docker/README_ubuntu.md

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+# Docker GRASS GIS (Ubuntu Linux)
+
+Dockerfile with an [Ubuntu Linux](https://ubuntu.com/) image with [GRASS GIS](https://grass.osgeo.org/), [PDAL](https://pdal.io) support and [grass-session](https://github.com/zarch/grass-session/).
+
+Download size of this image is of approximately 2.6 GB.
+
+Build the docker with:
+
+```bash
+$ sudo docker build --build-arg GRASS_VERSION=7.8 \
+                    --build-arg PYTHON_VERSION=3  \
+                    --build-arg PROJ_VERSION=4.9.3 \
+                    --build-arg PROJ_DATUMGRID_VERSION=1.8 \
+                    --file Dockerfile_ubuntu_pdal .
+```
+
+View the images available using `sudo docker images` and open a bash terminal with:
+
+```bash
+$ sudo docker run -i -t 15550df91610 /bin/bash
+bash-5.0#
+```