CI: always build and push docker images

[lukaszstolarczuk]

On stable branches we don't commit much, so this will hardly impact
usage of our CI, and since we always rebuild, script 'pull-or-rebuild.sh'
is no longer necessary.

It's an alternative for #929 :

• it removes dead code from the stable branch,
• it simplifies read of the workload,
• but it will require to clean stable branch each time after release (RELEASE_STEPS.md will be updated if we choose this option).

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This change is 

[karczex]

Reviewable status: 0 of 2 files reviewed, all discussions resolved

[codecov]

Codecov Report

Merging #935 into stable-1.5 will increase coverage by 0.11%.
The diff coverage is n/a.

@@              Coverage Diff               @@
##           stable-1.5     #935      +/-   ##
==============================================
+ Coverage       90.32%   90.43%   +0.11%     
==============================================
  Files              26       26              
  Lines            1023     1025       +2     
==============================================
+ Hits              924      927       +3     
+ Misses             99       98       -1     

Flag	Coverage Δ
#tests_clang_debug_cpp17	61.93% <ø> (?)
#tests_clang_release	61.46% <ø> (ø)
#tests_gcc_debug	93.40% <ø> (ø)
#tests_gcc_release_cpp17	?

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Impacted Files	Coverage Δ
include/libpmemobj++/detail/life.hpp	92.30% <0.00%> (+0.64%)	⬆️
include/libpmemobj++/make_persistent_atomic.hpp	90.90% <0.00%> (+0.90%)	⬆️
include/libpmemobj++/detail/make_atomic_impl.hpp	94.44% <0.00%> (+5.55%)	⬆️

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[karczex]

@lukaszstolarczuk
It should be possible to trigger server-side post-receive hook to cherry-pick some patches (like this one) on stable branch creation.

[igchor]

TBH, I'd prefer https://github.com/pmem/libpmemobj-cpp/pull/929/files

With this approach we'll have to resolve merge conflicts for each new stable branch when merging with master.

Reviewable status: 0 of 2 files reviewed, all discussions resolved

[lukaszstolarczuk]

#929 is most likely the way we go to solve our problem.