Schedule automatic image updates

ChangeLog:

	* .github/workflows/build-base.yml:
	* .github/workflows/build-latest.yml:
	* README.md:

Author: mattkretz

.github/workflows/build-base.yml

@@ -6,6 +6,8 @@ name: Build Base Image
 
 on:
   workflow_dispatch:  # Allows manual trigger via GitHub UI
+  schedule:
+    - cron: '0 3 1 * *'  # At 03:00 UTC every 1st day of the month
 
 permissions:
   contents: read

.github/workflows/build-latest.yml

@@ -6,6 +6,8 @@ name: Build Latest Image
 
 on:
   workflow_dispatch:  # Allows manual trigger via GitHub UI
+  schedule:
+    - cron: '0 4 * * 1'  # At 04:00 UTC every Monday
 
 permissions:
   contents: read

README.md

@@ -74,3 +74,9 @@ jobs:
           CXX: clang++-${{ matrix.version }}
         run: make check
 ```
+
+## Updates
+
+The images are automatically updated once per month (base) / once per week 
+(latest). There may also be manual updates whenever the feature set needs to 
+change.