Start milestone 3

- Add functionality to query online sources (Keyservers, etc) for
  certificates.
- Use introducers to determine which UserIDs should be fetched from
  online soources. Check certifications before importing certifiactes
  into the local cert store.
- start docker image for local end-to-end testing.

docker/README

@@ -0,0 +1,31 @@
+# Content
+
+This directory contains docker images and related provisioning files for setting
+up test environments.
+
+## MTA
+- docker container file: MTAContainerFile
+- provisioning: inside mta_provision/
+
+### Build
+```
+$ docker build -f MTAContainerFile -t localhost/mta .
+```
+
+### Use
+
+Check the image interactively
+```
+$ docker run -ti --rm localhost/mta bash -i
+```
+
+Use locally, no git clone and cargo build. This use case assumes that a binary
+has been locally build and can be mapped into the docker image.
+
+The mapped binary will be started, alongside `postfix` and `hockeypuck`.
+
+```
+$ docker run -ti --rm \
+  --mount type=bind,src=$(pwd)/target/release,dst=/opt/husk/mapped/
+  localhost/mta bash -c "/usr/bin/prepare.sh && bash -i"
+```