README Getting Started ends with a signed image request (closes #357)

Getting Started now runs script/bootstrap --cgo before make build, says
that logging in at http://localhost:8080/ needs debug, and ends with a
signed /v1/image/ request for an image on any public HTTPS server. Its
curl exits non-zero when pixa refuses the request.

Model: opus-5-5

README.md

@@ -15,9 +15,10 @@ In this document, the upstream host is the server that holds an original image.
 ## Getting Started
 
 ```bash
-# clone and build
+# clone, install what compiling needs, and build
 git clone https://git.eeqj.de/sneak/pixa.git
 cd pixa
+script/bootstrap --cgo
 make build
 
 # make a config file from the example and put a real signing key in it
@@ -26,18 +27,30 @@ openssl rand -base64 32   # prints a new signing key
 $EDITOR config.yml        # replace the signing_key placeholder with it
 ./bin/pixad --config config.yml
 
-# or build and run the Docker image
+# or build and run the Docker image with that signing key
 make docker
-docker run -p 8080:8080 -e PIXA_SIGNING_KEY="$(openssl rand -base64 32)" pixa:latest
+docker run -p 8080:8080 -e PIXA_SIGNING_KEY='the new signing key' pixa:latest
 ```
 
-`make build` compiles pixa on your machine into `bin/pixad`. `pixad` is the
-server program. Compiling needs the libraries `script/bootstrap --cgo` installs
-(see Deployment).
+`script/bootstrap --cgo` installs Go, a C compiler and the image libraries pixa
+needs (see Deployment). `make build` then compiles pixa on your machine into
+`bin/pixad`. `pixad` is the server program.
 
 The signing key is pixa's one required secret. It must be 32 characters or
 longer. The placeholder in the example file is refused at startup.
 
+A container takes its settings from environment variables (see Configuration).
+Only `PIXA_SIGNING_KEY` is required. Without it, the container exits at startup
+and names the variable. Every other setting has a built-in default. A config
+file mounted at `/etc/pixa/config.yml` is read when present. An environment
+variable wins over the same setting in the file.
+
+pixa's login page is at `http://localhost:8080/`. Logging in there works only
+with `debug: true` in `config.yml`, or `-e PIXA_DEBUG=true` for the container.
+Otherwise pixa refuses the login with 403, because the page was opened over
+plain HTTP. Use `debug` only on your own machine. A pixa that others use serves
+its pages over HTTPS through a reverse proxy (see Deployment).
+
 Once pixa runs, it serves images through URLs of this form:
 
 ```
@@ -60,11 +73,25 @@ The example config allowlists no host. Every image on an allowlisted host is
 served to anyone, without a signature, at any size and in any format. So
 allowlist only hosts you control, where nobody else can put images.
 
-A container takes its settings from environment variables (see Configuration).
-Only `PIXA_SIGNING_KEY` is required. Without it, the container exits at startup
-and names the variable. Every other setting has a built-in default. A config
-file mounted at `/etc/pixa/config.yml` is read when present. An environment
-variable wins over the same setting in the file.
+To see pixa serve an image, sign a URL for an image on any public HTTPS server.
+Run this in a second terminal while pixa runs, with your own signing key and
+image:
+
+```bash
+key='the new signing key'
+image_host=cdn.example.com    # a public HTTPS server
+image_path=/photos/cat.jpg    # an image on it
+exp=$(($(date +%s) + 3600))   # the URL expires in one hour
+sig=$(printf '%s' "$image_host:$image_path::400:300:webp:$exp:85:cover" |
+    openssl dgst -sha256 -hmac "$key" -binary | base64 | tr '+/' '-_')
+curl --fail-with-body -o cat.webp \
+    "http://localhost:8080/v1/image/$image_host$image_path/400x300.webp?sig=$sig&exp=$exp"
+```
+
+`cat.webp` is then the image at 400x300. If pixa refuses the request, curl
+reports an error and `cat.webp` holds pixa's error message instead. The image
+path must not hold a `%`, a `:` or a query. Signed URLs explains how to sign
+those.
 
 ## Deployment