Add readme and rename Dockerfile.

readme.md

@@ -0,0 +1,24 @@
+# Build instructions
+
+Run `docker build .` in this directory. After the build succeeds, note the image hash.
+
+
+# Troubleshooting
+
+If you get any error during the build, try pulling the required base images prior to building:
+```commandline
+docker image pull gentoo/portage:latest
+docker image pull gentoo/stage3:amd64-nomultilib-openrc
+```
+
+# Publishing
+Tag the build image with the current date as the name:
+```commandline
+docker image tag <hash> fritteli/pkgcheck:yyyyMMdd
+docker image tag <hash> fritteli/pkgcheck:latest
+```
+Push the tagged images:
+```commandline
+docker image push fritteli/pkgcheck:yyyyMMdd
+docker image push fritteli/pkgcheck:latest
+```