Add support for update, install arguments

- 'install' takes a list of packages to install with apt
- 'update' triggers an 'apt update' before the run
- setting 'install' will force an update regardless of the 'update' value
- 'entrypoint.py' is made unbuffered (by running 'python3 -u') to
  more accurately interleave the output of various commands

action.yml

@@ -21,6 +21,17 @@ inputs:
       dependencies, for example with
       "apt-get update -y && apt-get install -y perl"
     required: false
+  update:
+    description:
+      Set this flag to run 'apt update' in the container before starting
+      the sphinx build.
+    required: false
+    default: false
+  install:
+    description:
+      A list of additional 'apt' packages to install before running
+      sphinx. (Setting this will force-enable 'update' as well.)
+    required: false
 runs:
   using: 'docker'
-  image: 'Dockerfile'
+  image: 'Dockerfile'

entrypoint.py

@@ -1,14 +1,36 @@
-#!/usr/bin/env python3
+#!/usr/bin/env -S python3 -u
 import os
 import json
+import subprocess
+import sys
 from sphinx_action import action
 
 # This is the entrypoint called by Github when our action is run. All the
 # Github specific setup is done here to make it easy to test the action code
 # in isolation.
+
+def interpret_env(env_var):
+    if isinstance(env_var, str):
+        return env_var.lower() not in ["false", "no", "off" "0"]
+    return bool(env_var)
+
 if __name__ == "__main__":
     print("[sphinx-action] Starting sphinx-action build.")
 
+    update = os.environ.get("INPUT_UPDATE", False)
+    should_update = interpret_env(update)
+
+    install = os.environ.get("INPUT_INSTALL", "")
+    packages = install.split()
+
+    if should_update or packages:
+        print("Updating apt...")
+        subprocess.call(["/usr/bin/apt", "-y", "update"])
+
+    print(f"{len(packages)} packages (plus dependencies) to install")
+    if packages:
+        subprocess.call(["/usr/bin/apt", "-y", "install", *packages])
+
     if "INPUT_PRE-BUILD-COMMAND" in os.environ:
         pre_command = os.environ["INPUT_PRE-BUILD-COMMAND"]
         print("Running: {}".format(pre_command))