diff --git a/.dockerignore b/.dockerignore
index 5b2701b..376fa60 100644
--- a/.dockerignore
+++ b/.dockerignore
@@ -1,8 +1,31 @@
 # .git is sent so that `make build` in the Dockerfile can stamp the commit into
 # upaas. List no tracked file here: git would see it as deleted in the build and
 # the version would end in -dirty.
-.env
-bin/
-.vscode/
-.idea/
-*.test
+
+# The patterns of .gitignore; **/ makes Docker match them in every directory.
+**/.DS_Store
+**/Thumbs.db
+**/*.swp
+**/*.swo
+**/*~
+**/*.bak
+**/.idea/
+**/.vscode/
+**/*.sublime-*
+**/node_modules/
+**/.env
+**/.env.*
+**/*.pem
+**/*.key
+**/bin/
+**/*.exe
+**/*.exe~
+**/*.dll
+**/*.so
+**/*.dylib
+**/*.test
+**/*.out
+/data/
+
+# Git never applies its ignore patterns inside .git; send all of it again.
+!.git/**
diff --git a/.gitignore b/.gitignore
index 69191af..5e9024a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,5 @@
+# .dockerignore repeats these patterns; change both together.
+
 # OS
 .DS_Store
 Thumbs.db
@@ -29,3 +31,6 @@ bin/
 *.dylib
 *.test
 *.out
+
+# upaasd's data directory when it runs from the checkout (UPAAS_DATA_DIR default)
+/data/
diff --git a/TODO.md b/TODO.md
index d6bca11..7482492 100644
--- a/TODO.md
+++ b/TODO.md
@@ -20,6 +20,11 @@ regress.
 
 # Completed Steps
 
+- 2026-10-02: `docker build .` no longer sends git-ignored files, such as
+  `.env.local`, `*.key` files or upaasd's `data/` directory with its session
+  key, into the build stages and the build cache: `.dockerignore` now leaves out
+  everything `.gitignore` does, and `data/` is git-ignored (#266).
+
 - 2026-10-02: The build no longer passes the CPU architecture in: upaas reads it
   from Go's `runtime.GOARCH` when it runs, and the startup log line reports it
   as `arch`. `CONVENTIONS.md` follows the updated conventions in `sneak/prompts`