ci: Publish Docker image to ghcr.io on release (#82)

Adds a workflow that builds and publishes a Docker image
to ghcr.io based on the Dockerfile in the root of the repository.

This will become available at
https://github.com/uber-go/sally/pkgs/container/sally.
Users will be able to import it by using `ghcr.io/uber-go/sally:$tag`
with `docker pull` or in the `FROM` clause of their own Docker file.

As a test for this, I published it manually to my own fork.
The image is at https://github.com/abhinav/sally/pkgs/container/sally.
With that, I ran the following Dockerfile:

    FROM ghcr.io/abhinav/sally:master
    COPY sally.yaml /

And it worked as expected.

1 files changed, 5 insertions, 0 deletions

diff --git a/README.md b/README.md
index 92a8da1..125cd19 100644
--- a/README.md
+++ b/README.md
@@ -5,10 +5,15 @@ to serve vanity import paths for Go modules.
 
 ## Installation
 
+To build sally from source, use:
+
 ```bash
 go install go.uber.org/sally@latest
 ```
 
+Alternatively, get a pre-built Docker image from
+https://github.com/uber-go/sally/pkgs/container/sally.
+
 ## Usage
 
 Create a YAML file with the following structure: