build: add a make build target, drop README's raw go build (closes #19)

The executable builds to build/rogue. All generated artifacts go under
build/, which .gitignore covers as a whole; a target writing outside it
can commit its output.

build is in neither check nor test: check stays fmt-check lint test and
still writes nothing into the working tree.

README's "Building and running" block and the run examples use
./build/rogue, and no raw go invocation is left in the file.

Verified: make build writes build/rogue and git status stays clean;
GOFLAGS=-count=1 make check green in 34s with the lint layer executing
(21.9s, "0 issues.", not CACHED) and the suite running for real (cmd/rogue
1.027s, game 3.450s), git status clean afterwards.

.dockerignore

@@ -2,3 +2,7 @@
 # excluding a self-contained Go source here drops it from the lint silently.
 # Never exclude Go sources, go.mod/go.sum or .golangci.yml.
 .git
+
+# Generated artifacts only; `make build` puts a multi-megabyte binary here
+# and it would otherwise be shipped into the build context.
+/build/