add "rebuild" param to utils' script, to force dockers rebuilding

[lukaszstolarczuk]

it makes more sense to rebuild dockers all the time on stable branches,
because we don't have to count on DockerHub's policy to keep our
docker images on their servers.

this change is based on commit on master: 0c5bf91

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This change is 

[codecov]

Codecov Report

Merging #929 into stable-1.5 will decrease coverage by 31.01%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           stable-1.5     #929       +/-   ##
===============================================
- Coverage       90.32%   59.31%   -31.02%     
===============================================
  Files              26      101       +75     
  Lines            1023     6697     +5674     
===============================================
+ Hits              924     3972     +3048     
- Misses             99     2725     +2626     

Flag	Coverage Δ
#tests_clang_release	54.92% <ø> (-6.54%)	⬇️
#tests_gcc_debug	58.13% <ø> (-35.27%)	⬇️
#tests_gcc_release_cpp17	?

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Impacted Files	Coverage Δ
include/libpmemobj++/detail/pexceptions.hpp
include/libpmemobj++/pext.hpp
...lude/libpmemobj++/make_persistent_array_atomic.hpp
include/libpmemobj++/p.hpp
include/libpmemobj++/make_persistent.hpp
include/libpmemobj++/experimental/array.hpp
include/libpmemobj++/utils.hpp
include/libpmemobj++/condition_variable.hpp
include/libpmemobj++/mutex.hpp
include/libpmemobj++/detail/conversions.hpp
... and 117 more

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[KFilipek]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[igchor]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[lukaszstolarczuk]

it's an alternative to #935 :

• it uses "rebuild", which is already available on the master (if we choose the other option we should update other_OSes.yml to also not use this flag and rather just use build-image.sh script),
• it requires less work after release (just add "rebuild" param in the CI workflow),
• but it leaves dead code on the stable branches.