Dockerfile: Use python3 executable.

Python should be invoked as `python3` since we are writing python3 code
and not python2. See https://peps.python.org/pep-0394/#recommendation
Many (most?) distributions do not distribute a "python" executable any
more, forcing you to pick either 2 or 3.

.containerversion

@@ -1 +1 @@
-42
+43

Dockerfile

@@ -117,9 +117,6 @@ RUN if [ "${TARGETPLATFORM}" = "linux/arm64" ]; then \
     rm /tmp/protoc-21.2.zip
 ENV PATH /opt/protoc/bin:$PATH
 
-# Make Python3 the default
-RUN update-alternatives --install /usr/bin/python python /usr/bin/python3 1
-
 # Developer tools
 RUN apt-get update && apt-get install -y \
     bash-completion

external/CMakeLists.txt

@@ -52,6 +52,7 @@ ExternalProject_Add(libwally-core
   COMMAND         ${CMAKE_COMMAND} -E env
                   "CFLAGS=${LIBWALLY_CFLAGS}"
                   "LDFLAGS=${LIBWALLY_LDFLAGS}"
+                  PYTHON_VERSION=3
                   ${CMAKE_CURRENT_SOURCE_DIR}/libwally-core/configure
                   ${CONFIGURE_FLAGS}
                   ${LIBWALLY_CONFIGURE_FLAGS}

scripts/expand_template

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 """Expand python templates"""
 
 from string import Template

scripts/get_version

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 """
 Parse the tags and strip the prefix
 First argument should be prefix to match against