testing: update the python versions in the Dockerfile

[tmatsuo]

also enable the Docker image upload in python 3.8 periodic build.

fixes#4126

[tmatsuo]

Doh. I overlooked there is python 2.7.18

[tmatsuo]

The current logic for installing Python seems flaky. I think it fails to import gpg keys.
Also gpg --verify ... returns 0 even if there's no public key.

The tests in py-3.7 builds are all skipped because there's no python-3.7
Some tests in py-2.7 builds failed because there's non python.h for python2.7.

I'll modify the python installation to fail when key verification fails.

Update:

Actually gpg --verify returns non-zero exit code, but because gpg --verify is in for loop, it doesn't immediately exit with non-zero status. Instead, the for loop exits with non-zero status code. Because we set set -ex in the for loop, the outer for loop doesn't exit because it is not affected by set -e.

I changed the order of the script and now it should fail upon signature verification failures.

[tmatsuo]

py-2.7 build failed with several errors with "bad zip files". I'll restart it.

[tmatsuo]

The builds are all green, merging