Add a `make shell` rule to run a shell

Makefile

@@ -98,6 +98,22 @@ bin/$(ARCH)/$(BIN): build-dirs
 	        ./build/build.sh                                                \
 	    "
 
+# Example: make shell CMD="-c 'date > datefile'"
+shell: build-dirs
+	@echo "launching a shell in the containerized build environment"
+	@docker run                                                             \
+	    -ti                                                                 \
+	    --rm                                                                \
+	    -u $$(id -u):$$(id -g)                                              \
+	    -v "$$(pwd)/.go:/go"                                                \
+	    -v "$$(pwd):/go/src/$(PKG)"                                         \
+	    -v "$$(pwd)/bin/$(ARCH):/go/bin"                                    \
+	    -v "$$(pwd)/bin/$(ARCH):/go/bin/$$(go env GOOS)_$(ARCH)"            \
+	    -v "$$(pwd)/.go/std/$(ARCH):/usr/local/go/pkg/linux_$(ARCH)_static" \
+	    -w /go/src/$(PKG)                                                   \
+	    $(BUILD_IMAGE)                                                      \
+	    /bin/sh $(CMD)
+
 DOTFILE_IMAGE = $(subst :,_,$(subst /,_,$(IMAGE))-$(VERSION))
 
 container: .container-$(DOTFILE_IMAGE) container-name