Added Docker support.

Added a Dockerfile to allow building Shrek into an image. It uses Alpine
Linux to keep the image size small. Shrek doesn't use any networking so
there shouldn't be any CGO issues.

I also created a Docker Hub repository for Shrek so users have the
convenience of running Shrek inside Docker without having to build the
image themselves.

The README has been updated to include Docker instructions.

refs #3

Author: innix

.dockerignore

@@ -0,0 +1,15 @@
+**/.dockerignore
+**/.env
+**/.git
+**/.gitattributes
+**/.gitignore
+**/.idea
+**/.vscode
+**/bin
+**/build
+**/deploy
+**/docker-compose*
+**/Dockerfile*
+**/releases
+assets
+examples

Dockerfile

@@ -0,0 +1,24 @@
+FROM golang:1.17-alpine AS builder
+
+WORKDIR /usr/src/shrek
+
+# Pre-copy/cache go.mod for pre-downloading dependencies and only re-downloading
+# them in subsequent builds if they change.
+COPY go.mod go.sum ./
+RUN go mod download && go mod verify
+
+# Copy all other project files.
+COPY . .
+
+# Build app.
+RUN go build -v -o /usr/local/bin/shrek ./cmd/shrek
+
+FROM alpine:latest AS final
+
+WORKDIR /app
+
+# Copy compiled binary into final image.
+COPY --from=builder /usr/local/bin/shrek .
+
+# Define entry point.
+ENTRYPOINT ["/app/shrek", "-d", "/app/generated/"]

README.md

@@ -57,6 +57,58 @@ To see full usage, use the help flag `-h`:
 shrek -h
 ```
 
+# Running in Docker
+
+[![Docker Hub][dkhub-badge]][dkhub-page]
+
+You can run Shrek in Docker by pulling a pre-built image from [Docker Hub][dkhub-page]
+or by building an image locally.
+
+## Option 1: Pull image from Docker Hub
+
+There's no need to run `docker pull` directly, because `docker run` will automatically
+download the image from Docker Hub if it isn't found locally. So you can save a step
+and run Shrek with a one-liner:
+
+```bash
+docker run --rm -it \
+    -v "$PWD/generated":/app/generated \
+    innix/shrek:latest \
+    -n 3 food:ad barn:yd
+```
+
+## Option 2: Build image locally
+
+Build the Docker image locally using the [`Dockerfile`](Dockerfile):
+
+```bash
+docker build -t shrek:latest .
+```
+
+Then run a new container using the locally built image:
+
+```bash
+docker run --rm -it \
+    -v "$PWD/generated":/app/generated \
+    shrek:latest \
+    -n 3 food:ad barn:yd
+```
+
+## How does the `docker run` command work with Shrek?
+
+When running Shrek in a container, you pass arguments to it by placing them at the
+very end of the `docker run` command. In the example commands above, the arguments
+`-n 3 food:ad barn:yd` are passed to the Shrek binary.
+
+The Docker image configures Shrek to save found `.onion` addresses to the directory
+`/app/generated/` in the container's file system. To access that directory from the
+host, you need to create a shared volume using Docker's `-v` argument. In the above
+examples, the `-v` argument will allow the host to access the container's directory
+by browsing to `$PWD/generated`.
+
+If you're getting a permission error when trying to access the `generated` directory
+on the host, take a look at [this FAQ][docker-access-dir-faq].
+
 # Using Shrek as a library
 
 You can use Shrek as a library in your Go code. Add it to your `go.mod` file by running
@@ -181,6 +233,17 @@ func main() {
 ```
 </details>
 
+## Why can't I access the `generated` directory created by the Docker container?
+
+If the directory (or any of the sub-directories) were created by the Shrek process
+running in the Docker container, they will be owned by the `root` user that is running
+the Shrek process. You need to change ownership of the directory back to your user,
+which can be done by running the [`chown`][chown.1-page] command:
+
+```bash
+sudo chown -R $USER "$PWD/generated"
+```
+
 ## Why "Shrek"?
 
 Onions have layers, ogres have layers.
@@ -209,5 +272,11 @@ Shrek is distributed under the terms of the MIT License (see [LICENSE](LICENSE))
 [licen-badge]: <https://img.shields.io/github/license/innix/shrek?style=for-the-badge>
 [licen-page]: <LICENSE> "Project License"
 
+[dkhub-badge]: <https://img.shields.io/docker/v/innix/shrek?color=2496ed&label=Docker%20Hub&logo=docker&style=for-the-badge>
+[dkhub-page]: <https://hub.docker.com/r/innix/shrek> "innix/shrek - Docker Hub page"
+
 [mkp224o-page]: <https://github.com/cathugger/mkp224o> "cathugger/mkp224o - GitHub page"
 [ghbine-page]: <https://github.com/cretz/bine/> "cretz/bine - GitHub page"
+[chown.1-page]: <https://linux.die.net/man/1/chown> "chown(1) - Linux man page"
+
+[docker-access-dir-faq]: <#why-cant-i-access-the-generated-directory-created-by-the-docker-container> "Why can't I access the generated directory created by the Docker container?"