[bug] Duplication of __openwisp_version__ leads to stale info

[nemesifier]

Describe the bug
The version number __openwisp_version__ defined in images/common/openwisp/__init__.py is hardcoded and never updated during the release process, causing it to become stale.

Currently, it shows version 25.10.0 while the latest release is 25.10.2 (I am going to fix this shortly).

This creates version duplication across the codebase.

Steps To Reproduce

1. Check the current version in images/common/openwisp/__init__.py
2. Compare it with the latest GitHub release tag
3. Observe that the version in __init__.py is outdated (e.g., 25.10.0 vs 25.10.2)

Expected behavior

I am not sure what's the best way to deal with this right now and I am open for suggestions.

We either need to automatically update this as part of the release process, or we need to make this dynamic and get the right version from the information we already have.

[Eeshu-Yadav]

To resolve the stale __openwisp_version__ definition we can dynamically pass OPENWISP_VERSION from the Makefile (the source of truth) into the Python scope.

Here is the proposed solution:

1. Update base-build in the Makefile to pass --build-arg OPENWISP_VERSION=$(OPENWISP_VERSION).
2. Update images/openwisp_base/Dockerfile to accept the ARG and assign it as an ENV variable (OPENWISP_VERSION=${OPENWISP_VERSION}).
3. In images/common/openwisp/__init__.py, dynamically fetch the version reading from the OS environment (os.environ.get("OPENWISP_VERSION", "unknown")).

This will eliminate the hardcoded version in Python and leverage the existing variable managed in the project root. Let me know if you are open to this approach and I can go ahead

[nemesifier]

@Eeshu-Yadav we need to take into account the changes being worked on in #555. Let's wait until that PR is merged.