docker: make build- and push-image scripts more verbose

Merge stable-1.5 into stable-1.6

otherwise it will still use found (most likely, newer) clang-format.

Update doxygen settings

bump docker images versions

and set clang-format to fixed version (6.0).

fixes #854

Merge stable-1.6 into stable-1.7

Merge stable 1.5 into stable 1.6

contigous_iterator: make operator[] take signed integral instead

of unsigned.

The reason is that Windows cannot decide for a call like this:

`it[0] = 10`

whether to use:
* operator[](unsigned) -- one conversion from signed to unsigned
* operator T*() and then build in operator[](signed) -- one conversion

Merge stable-1.6 into stable-1.7

Merge stable-1.5 into stable-1.6

fix dir creation in auto docs update

Only parse libpmemobj version if it matches a pattern (major[.]minor.*)

doc: enable auto gh-pages generation in docker script

Ref: https://github.com/pmem/libpmemobj-cpp/issues/377

Add skip flags for additional artifacts built/installed in dockerimages

Merge stable-1.6 into stable-1.7

Create separate Docker images for stable-1.7 branch

Create separate Docker images for stable-1.6 branch

Add missing pmdk-debuginfo rpms installation

Merge changes from stable-1.6

tests: remove unnecessary #include <libpmemobj.h>