.dockerignore: ignore unnecessary repo files (#54)

On the penultimate line of our Dockerfile we have the instruction:

    COPY . .

which previously copied some small unneeded repo files into the images.
Add them to the ignore list so that they are no longer copied.

With this commit, the /opt/test-runner directory in the image contains
only this file:

    /opt/test-runner/bin/run.sh

Refs: #34

Author: ee7

.dockerignore

@@ -1,8 +1,12 @@
 .git/
 .appends/
 .github/
+.dockerignore
 .gitattributes
 .gitignore
+CODE_OF_CONDUCT.md
+LICENSE
+README.md
 Dockerfile
 bin/run-in-docker.sh
 bin/run-tests-in-docker.sh