Redo Dockerfile

- Build the currently checked-out code, instead of cloning the repo
inside the Dockerfile. This makes it much easier to build a container
for a particular branch or commit; people working on personal forks
will be able to build containers for their forks without modifying the
Dockerfile.

- Switch from Alpine to distroless; I couldn't actually get the current
version of the Dockerfile to build, it kept dying with some error about
gvisor. Aside from building with no trouble, the new Dockerfile reduces
the size of the image from 23MB to 9MB.

- Move Dockerfile into the root; this is a matter of taste, but allows
one to simply `docker build` the directory instead of having to also
specify the path to the Dockerfile. As part of this, I removed the
`config` and `Makefile` from the `docker` directory, since they seemed
specific to someone's setup and incomplete without that context.

Dockerfile

@@ -0,0 +1,15 @@
+# Start by building the application.
+FROM golang:1.18 as build
+
+WORKDIR /usr/src/wireproxy
+COPY . .
+
+RUN CGO_ENABLED=0 go build ./cmd/wireproxy
+
+# Now copy it into our base image.
+FROM gcr.io/distroless/static-debian11:nonroot
+COPY --from=build /usr/src/wireproxy/wireproxy /usr/bin/wireproxy
+
+VOLUME [ "/etc/wireproxy"]
+ENTRYPOINT [ "/usr/bin/wireproxy" ]
+CMD [ "--config", "/etc/wireproxy/config" ]