Add docker-standard ftp_proxy/FTP_PROXY env var, and socks-standard all_proxy/ALL_PROXY

Signed-off-by: Avi Deitcher <avi@deitcher.net>

docs/packages.md

@@ -151,3 +151,27 @@ linuxkit pkg build -org=wombat -disable-content-trust -hash=foo push
 
 and this will create `wombat/<image>:foo-<arch>` and
 `wombat/<image>:foo` for use in your YAML files.
+
+### Proxies
+
+If you are building packages from behind a proxy, `linuxkit pkg build` respects
+the following environment variables, and will set them as `--build-arg` to
+`docker build` when building a package.
+
+* `http_proxy` / `HTTP_PROXY`
+* `https_proxy` / `HTTPS_PROXY`
+* `ftp_proxy` / `FTP_PROXY`
+* `no_proxy` / `NO_PROXY`
+* `all_proxy` / `ALL_PROXY`
+
+Note that the first four of these are the standard built-in `build-arg` options available
+for `docker build`; see the [docker build documentation](https://docs.docker.com/v17.09/engine/reference/builder/#arg).
+The last, `all_proxy`, is a standard var used for socks proxying. Since it is not built into `docker build`,
+if you want to use it, you will need to add the following line to the dockerfile:
+
+```dockerfile
+ARG all_proxy
+```
+
+Linuxkit does not judge between lower-cased or upper-cased variants of these options, e.g. `http_proxy` vs `HTTP_PROXY`,
+as `docker build` does not either. It just passes them through "as-is".

src/cmd/linuxkit/pkglib/docker.go

@@ -41,13 +41,19 @@ func isExecErrNotFound(err error) bool {
 	return eerr.Err == exec.ErrNotFound
 }
 
+// these are the standard 4 build-args supported by `docker build`
+// plus the all_proxy/ALL_PROXY which is a socks standard one
 var proxyEnvVars = []string{
 	"http_proxy",
 	"https_proxy",
 	"no_proxy",
+	"ftp_proxy",
+	"all_proxy",
 	"HTTP_PROXY",
 	"HTTPS_PROXY",
 	"NO_PROXY",
+	"FTP_PROXY",
+	"ALL_PROXY",
 }
 
 func (dr dockerRunner) command(args ...string) error {
@@ -72,7 +78,12 @@ func (dr dockerRunner) command(args ...string) error {
 					[]string{"--build-arg", fmt.Sprintf("%s=%s", proxyVarName, value)}...)
 			}
 		}
-		cmd.Args = append(append(cmd.Args[:2], buildArgs...), cmd.Args[2:]...)
+		// cannot use usual append(append( because it overwrites part of it
+		newArgs := make([]string, len(cmd.Args)+len(buildArgs))
+		copy(newArgs[:2], cmd.Args[:2])
+		copy(newArgs[2:], buildArgs)
+		copy(newArgs[2+len(buildArgs):], cmd.Args[2:])
+		cmd.Args = newArgs
 
 		if dr.ctx != nil {
 			stdin, err := cmd.StdinPipe()