help run mox with docker

in the Dockerfile, allow running on privileged ports and expose those ports.

add a docker-compose.yml with instructions for the quickstart.

fix running imaptest somewhat. after a short while it will hit the rate limiter.

in quickstart, recognize we are running under docker, and print slightly
different commands to set permissions, and skip generating the systemd service
file. als fix cleaning up the right paths during failure in quickstart.

for issue #3

Dockerfile.moximaptest

@@ -0,0 +1,11 @@
+FROM golang:1-alpine AS build
+WORKDIR /build
+COPY . .
+RUN GOPROXY=off CGO_ENABLED=0 go build -trimpath
+
+# Using latest may break at some point, but will hopefully be convenient most of the time.
+FROM alpine:latest
+WORKDIR /mox
+COPY --from=build /build/mox /bin/mox
+
+CMD ["/bin/mox", "serve"]