Run the linter only in Docker (closes #20)

script/lint now builds only the lint stage of the Dockerfile, without
the build cache, so every run executes the linter; the image is tagged
simplelog-lint. The lint stage calls golangci-lint directly, since make
lint is itself a docker build of that stage. script/cibuild and
script/docker also build without the cache, so their check steps always
run. script/fmt no longer runs golangci-lint --fix, and script/bootstrap
no longer installs it. golangci-lint config verify is left out, on the
owner's ruling. The README, TODO.md and the script/cibuild comment say
what now runs.

Model: opus-5-5

script/docker

@@ -1,6 +1,7 @@
 #!/bin/sh
 # script/docker: build the Docker image tagged with the project name.
 # Identical in all repos; the tag comes from script/projectname.
+# --no-cache because a cached check step is a check that did not run.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -8,7 +9,7 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build -t "$("$SCRIPT_DIR/projectname")" .
+    docker build --no-cache -t "$("$SCRIPT_DIR/projectname")" .
 }
 
 main "$@"