Explicitly provide Python toolchain, provide `python` in build container

This fixes being able to run py_binary targets within the build
container.

Each py_binary creates a stub that always has the #!/usr/bin/env python
shebang, and as such we need to have `python` available in the build
container. The stub then dispatches into the right Python interpreter,
which we now configure explicitely via rules_python's py_runtime_pair.

Test Plan: nothing breaks, future uses of py_binary (eg D389) will make actual use of this

X-Origin-Diff: phab/D390
GitOrigin-RevId: 78b6c51f09c720a46fbe2e6cbadb2a97d1161f7b
diff --git a/build/Dockerfile b/build/Dockerfile
index e07e60d..7332d90 100644
--- a/build/Dockerfile
+++ b/build/Dockerfile
@@ -32,7 +32,8 @@
 	postgresql \
 	expect \
 	grpc-cli \
-	nc
+	nc \
+	python-unversioned-command
 
 # Workaround for a binutils bugs in F30, which generates invalid ELF binaries
 # when linking statically with musl.