use copy instead of git clone in Dockerfiles

[mmacata]

This PR suggests to change the current behaviour of using git clone in the Dockerfiles. Before, the GRASS GIS version needed to be configured via ARG variables to clone the branch according to the wanted GRASS version. As the source code is here and next to the Dockerfiles already, a simple COPY can be used. The GRASS version then corresponds to the branch on which the Dockerimage is build.

Additionally a .dockerignore file was added to not copy the whole context on build time.

An example docker build can be done now with:

$ docker build \
         --file docker/debian/Dockerfile_debian_pdal \
         --tag grass-py3-pdal:latest-debian .

and to build a stable version:

$ git checkout remotes/origin/releasebranch_7_8
$ docker build \
         -f docker/debian/Dockerfile_debian_pdal \
         -t grass-py3-pdal:stable-debian .

Currently, this leads to the following flavours:

master:

• grass-py3-pdal:latest-ubuntu (GRASS GIS 7.9, proj=4.9.3, gdal=2.2.3, geos=3.6.2, sqlite=3.22.0, pdal 1.8.0, Python 3.6.8)
• grass-py3-pdal:latest-debian (GRASS GIS 7.9, proj=5.2.0, gdal=2.4.0, geos=3.7.1, sqlite=3.27.2, pdal 1.8.0, Python 3.7.3)
• grass-py3-pdal:latest-alpine (GRASS GIS 7.9, proj=5.2.0, gdal=2.4.0, geos=3.7.2, sqlite=3.29.0, pdal 1.9.1, Python 3.7.4)

releasebranch_7_8:

• grass-py3-pdal:stable-ubuntu (GRASS GIS 7.8.1, proj=4.9.3, gdal=2.2.3, geos=3.6.2, sqlite=3.22.0, pdal 1.8.0, Python 3.6.8)
• grass-py3-pdal:stable-debian (GRASS GIS 7.8.1, proj=5.2.0, gdal=2.4.0, geos=3.7.1, sqlite=3.27.2, pdal 1.8.0, Python 3.7.3)
• grass-py3-pdal:stable-alpine (GRASS GIS 7.8.1, proj=5.2.0, gdal=2.4.0, geos=3.7.2, sqlite=3.29.0, pdal 1.9.1, Python 3.7.4)

[neteler]

Looks good to me, thanks

[neteler]

Backported to relbranch78 in d328446