Update to python 3.7 (buster) and use build-args

The build arg can be used to either test different versions, but
also makes it easier to "grep" when upgrading versions.

The output format of `gpg2 --list-secret-keys` changed in the version
installed on Buster, so `grep` was replaced with `awk` to address
the new output format;

Debian Jessie:

    gpg2 --no-auto-check-trustdb --list-secret-keys
    /root/.gnupg/secring.gpg
    ------------------------
    sec   1024D/A7B21401 2018-04-25
    uid                  Sakuya Izayoi <[email protected]>
    ssb   1024g/C235E4CE 2018-04-25

Debian Buster:

    gpg2 --no-auto-check-trustdb --list-secret-keys
    /root/.gnupg/pubring.kbx
    ------------------------
    sec   dsa1024 2018-04-25 [SCA]
          9781B87DAB042E6FD51388A5464ED987A7B21401
    uid           [ultimate] Sakuya Izayoi <[email protected]>
    ssb   elg1024 2018-04-25 [E]

Signed-off-by: Sebastiaan van Stijn <[email protected]>
diff --git a/Dockerfile-docs b/Dockerfile-docs
index 105083e..9d11312 100644
--- a/Dockerfile-docs
+++ b/Dockerfile-docs
@@ -1,4 +1,6 @@
-FROM python:3.5
+ARG PYTHON_VERSION=3.7
+
+FROM python:${PYTHON_VERSION}
 
 ARG uid=1000
 ARG gid=1000