Image build pipeline should accept any tag/branch inside minor Qt versions

At moment, the `qt_docker.py` tool in `docker-images` repo accepts only certain, hardcoded branches to build qt. It should be modified to accept any tag, at least bugfix releases inside minor releases. For example, it should be allowed to build all `6.4.z` -versions without changes to that file whenever a new bugfix release is released by Qt.

There are some per-branch dependencies for external tools like emscripten and Android SDKs, but they usually change only between Qt minor versions, not between bugfix releases. Therefore these checks should be happen at the level of minor version.