Support {python} placeholder

This placeholder is expanded on *execution* of a container, rather than
on configuration/addition. This helps use the same Python installation
that is also executing the datalad-container code.

Previously, the docker-support code would expand and then hardcode
`sys.executable` on configuring a container. This led to non-portable
configuration (e.g., hardcoded `python.exe` on windows), and would fail
to pick up the correct python installation in any case where the
`python` entrypoint would not point to the correct one.

Closes datalad#226
Closes datalad#224

datalad_container/containers_add.py

@@ -65,7 +65,8 @@ def _guess_call_fmt(ds, name, url):
     elif url.startswith('shub://') or url.startswith('docker://'):
         return 'singularity exec {img} {cmd}'
     elif url.startswith('dhub://'):
-        return op.basename(sys.executable) + ' -m datalad_container.adapters.docker run {img} {cmd}'
+        # {python} is replaced with sys.executable on *execute*
+        return '{python} -m datalad_container.adapters.docker run {img} {cmd}'
 
 
 def _ensure_datalad_remote(repo):
@@ -142,6 +143,9 @@ class ContainersAdd(Interface):
             replaced with the desired command. Additional placeholders:
             '{img_dspath}' is relative path to the dataset containing the image,
             '{img_dirpath}' is the directory containing the '{img}'.
+            '{python}' expands to the path of the Python executable that is
+            running the respective DataLad session, for example a
+            'datalad containers-run' command.
             """,
             metavar="FORMAT",
             constraints=EnsureStr() | EnsureNone(),

datalad_container/containers_run.py

@@ -4,6 +4,7 @@
 
 import logging
 import os.path as op
+import sys
 
 from datalad.interface.base import Interface
 from datalad.interface.base import build_doc
@@ -115,6 +116,10 @@ def __call__(cmd, container_name=None, dataset=None,
                         'Convert it to a plain string.'.format(callspec))
             try:
                 cmd_kwargs = dict(
+                    # point to the python installation that runs *this* code
+                    # we know that it would have things like the docker
+                    # adaptor installed with this extension package
+                    python=sys.executable,
                     img=image_path,
                     cmd=cmd,
                     img_dspath=image_dspath,