major CI/utils cleanup

and make sure WORKDIR env is set.

it's useful for debugging, especially if tests fail.

Pushing image happens only on push events so tests are expected to pass.
Taking this into account - pushing can be done right after rebuilding
an image, instead of marking a flag for a later push.

- set proper IMG_VER on stable branches,
- use "rebuild" param (for pull-or-rebuild.sh) on stable branches.

rather than hardcoded address string.

and remove ndctl installation step in CentOS, since it's installed from package.

and use it in jobs not requiring image rebuilding.

instead of using array of hard-coded dir names.

fixes warning in log: 'Unable to find image '...' locally'.

so we don't have to update this piece of code every release.

and let other OSes build independently.

to make possible to override it for testing on real pmem. Default: ON.

to avoid warnings in build logs.

in run-doc-update.sh created PRs.

and additional minor cleanups in run-doc-update.sh

and don't build docs.

- tidy up workflow files,
- make scripts more verbose,
- unify printed messages,
- use proper ${} bash variables syntax,
- order code,
- remove redundant code.

Merge stable-1.11 into master

modify slice to work with output iterators

radix_tree: fix use after free

Iterator inside erase() cannot be incremented after calling delete_persistent.
Incrementing checks the value of leaf_->parent which means reading from free'd memory.

Merge stable-1.10 into stable-1.11

basic_string: remove workaround to compile under VC++2015

After added conversion operators there is no need
to manually cast string to string_view. Casting in this case
produces VC++2015 errors due to operator==() ambiguity.

Merge stable-1.9 into stable-1.10

docker: add Ubuntu-18.04 docker image and test it in other_OSes.yml

CI: use special environment var in GHA for Windows' PATH