Add a .dockerignore (closes #39)

Adds a .dockerignore, which REPO_POLICIES.md lists among the files every repo has. Until now every docker build sent the whole working tree as its build context, and the image's source archive is made from that context. It now keeps out .git, local build and test output, archives, local databases, .env and a local Go workspace. Every tracked file stays in, so the format check, the linter, the tests, the build and the source archive read the same files as before.

Without .git in the context the binary's build info records no git revision; nothing in routewatch reads it.

Model: opus-5-5

.dockerignore

@@ -0,0 +1,28 @@
+# Docker does not read .gitignore, and a pattern here matches from the root of
+# the build context only: a pattern meant for every directory needs `**/`.
+
+.git
+
+# Local build and debug output: `make build`, `make run`, `make asupdate`, test
+# binaries, coverage profiles and source archives.
+/bin
+/log.txt
+/out
+/pkg/asinfo/asdata.json
+**/*.tar.zst
+**/*.test
+**/*.out
+**/*.tmp
+
+# Local databases and secrets. The image carries a source archive of the whole
+# build context, so these would otherwise ship inside it.
+**/*.db
+**/*.db-journal
+**/*.db-wal
+**/.env
+
+# A local Go workspace points at directories outside the build context.
+/go.work
+/go.work.sum
+
+**/.DS_Store