generator: Add Dockerfile and Dockertag generator to produce similar containers with small differences

[iesahin]

The containers in this repository use the same codebase with small differences in tags etc.

Instead of creating a Dockerfile for each of these, we can automate the process by writing a generator script to produce these Dockerfiles. This will allow, for example, to produce a Dockerfile for each of the tags in https://github.com/iterative/dvc-get-started

build-all.bash script checks Dockerfile and Dockertag files in a directory. Before this, there can be a check for generate.bash in a directory to generate a set of Dockerfiles in different directories.