Add a Docker build, as another way of testing out hnix

.dockerignore

@@ -0,0 +1,7 @@
+.git
+Dockerfile
+Makefile
+README.md
+cabal.sandbox.config
+.cabal-sandbox/
+hnix.cabal

Dockerfile

@@ -0,0 +1,16 @@
+FROM lnl7/nix:2.0
+
+WORKDIR /tmp/build
+
+COPY default.nix /tmp/build
+COPY package.yaml /tmp/build
+
+# Install tools needed by builtins.fetchTarball, and then install all
+# dependencies into its own layer, which doesn't change.
+RUN nix-env -f '<nixpkgs>' -i gnutar gzip && \
+    nix-shell -Q -j2 --run true
+
+COPY . /tmp/build
+RUN nix-env -f . -i hnix
+
+CMD ["/root/.nix-profile/bin/hnix"]

README.md

@@ -24,6 +24,20 @@ $ cabal test
 $ LANGUAGE_TESTS=yes NIXPKGS_TESTS=yes cabal test
 $ ./dist/build/hnix/hnix --help
 ```
+
+## Building a Docker container
+
+If you don't have Nix installed, or you'd just like to play around with `hnix`
+completely separately from your main system, you can build a Docker container:
+
+```bash
+$ docker build -t hnix .
+$ docker run hnix hnix --eval --expr '1 + 2'
+
+# In order to refer to files under the current directory:
+$ docker run -v $PWD/:/tmp/build run hnix hnix default.nix
+```
+
 ## Building with full debug info
 
 To build `hnix` for debugging, and with full tracing output and stack traces,
@@ -33,7 +47,7 @@ use:
 $ nix-shell --arg doProfiling true
 $ cabal configure --enable-tests --enable-profiling --flags=tracing
 $ cabal build
-$ ./dist/build/hnix/hnix -v5 <args> +RTS -xc
+$ ./dist/build/hnix/hnix -v5 --trace <args> +RTS -xc
 ```
 
 Note that this will run quite slowly, but will give the most information as to