Add support for passing secrets as text (concourse#78)

Signed-off-by: Charles Duffy <[email protected]>

README.md

@@ -91,7 +91,7 @@ _(As a convention in the list below, all task parameters are specified with a
   DO_THING=false
   ```
 
-* `$BUILDKIT_SECRET_*`: extra secrets which are made available via
+* `$BUILDKIT_SECRET_*`: files with extra secrets which are made available via
   `--mount=type=secret,id=...`. See [New Docker Build secret information](https://docs.docker.com/develop/develop-images/build_enhancements/#new-docker-build-secret-information) for more information on build secrets.
 
   For example, running with `BUILDKIT_SECRET_config=my-repo/config` will allow
@@ -101,6 +101,13 @@ _(As a convention in the list below, all task parameters are specified with a
   RUN --mount=type=secret,id=config cat /run/secrets/config
   ```
 
+* `$BUILDKIT_SECRETTEXT_*`: literal text of extra secrets to be made available
+  via the same mechanism described for `$BUILDKIT_SECRET_*` above. The
+  difference is that this is easier to use with credential managers:
+
+  `BUILDKIT_SECRETTEXT_mysecret=(( mysecret ))` puts the content that
+  `(( mysecret ))` expands to in `/run/secrets/mysecret`.
+
 * `$IMAGE_ARG_*`: params prefixed with `IMAGE_ARG_*` point to image tarballs
   (i.e. `docker save` format) to preload so that they do not have to be fetched
   during the build. An image reference will be provided as the given build arg

cmd/build/main.go

@@ -3,8 +3,10 @@ package main
 import (
 	"bytes"
 	"encoding/json"
+	"io/ioutil"
 	"os"
 	"os/exec"
+	"path/filepath"
 	"strings"
 
 	"github.com/sirupsen/logrus"
@@ -17,6 +19,7 @@ const imageArgPrefix = "IMAGE_ARG_"
 const labelPrefix = "LABEL_"
 
 const buildkitSecretPrefix = "BUILDKIT_SECRET_"
+const buildkitSecretTextPrefix = "BUILDKIT_SECRETTEXT_"
 
 func main() {
 	req := task.Request{
@@ -58,6 +61,20 @@ func main() {
 
 			req.Config.BuildkitSecrets[seg[0]] = seg[1]
 		}
+
+		if strings.HasPrefix(env, buildkitSecretTextPrefix) {
+			seg := strings.SplitN(
+				strings.TrimPrefix(env, buildkitSecretTextPrefix), "=", 2)
+
+			// Q: Filter for environment variable names that are also legal shell variable names to disallow ../ etc?
+			secretDir := filepath.Join(os.TempDir(), "buildkit-secrets")
+			secretFile := filepath.Join(secretDir, seg[0])
+			err := os.MkdirAll(secretDir, 0700)
+			failIf("create secret directory", err)
+			err = ioutil.WriteFile(secretFile, []byte(seg[1]), 0600)
+			failIf("write to secret directory", err)
+			req.Config.BuildkitSecrets[seg[0]] = secretFile
+		}
 	}
 
 	logrus.Debugf("read config from env: %#v\n", req.Config)