docker: update readme (use latest tag)

[landam]

Readme files suggest to create Docker images for GRASS stable from releasebranch_8_2. The current stable branch is releasebranch_8_4. To avoid outdated instructions this PR suggests to checkout latest tag instead (completion of #2969)

The run example shows how to run grass binary instead of generic shell.

[echoix]

The title confused me at first, as I thought you were suggesting for users to use each docker image's "latest" tag, which we decided a while back that it didn't mean latest release, but latest and greatest commit (that I would call more like "beta").

However, the changes here are more of using git to return the latest (in the sense of last available git tag for a branch).

Also, it seems that before, the syntax of the commands were platform-agnostic, now they use command substitution, that works well for Linux (at least with bash), but for cmd.exe or powershell, won't work as is.