Tag 1.0.3 fails to build in docker

Trying to build "1.0.3" tag with plugins.

My composer file:
```yaml
services:
frontend:
build:
context: ./deps/caddy-docker/
dockerfile: Dockerfile-no-stats
args:
- plugins=locale,gzip
```

Error log:
```
github.com/mholt/certmagic
# github.com/mholt/certmagic
../certmagic/client.go:196:8: ht.ForceAttemptHTTP2 undefined (type *http.Transport has no field or method ForceAttemptHTTP2)
fetching missing dependencies: caddy, dnsproviders, hook.pluginloader
chdir /root/lib/caddy/gopath/src/github.com/caddyserver/caddy: no such file or directory
fetching missing dependencies: caddy, dnsproviders, hook.pluginloader
chdir /root/lib/caddy/gopath/src/github.com/caddyserver/caddy: no such file or directory
go get: warning: modules disabled by GO111MODULE=auto in GOPATH/src;
ignoring go.mod;
see 'go help modules'
github.com/mholt/certmagic
# github.com/mholt/certmagic
../certmagic/client.go:196:8: ht.ForceAttemptHTTP2 undefined (type *http.Transport has no field or method ForceAttemptHTTP2)
fetching missing dependencies: caddy, dnsproviders, hook.pluginloader
chdir /root/lib/caddy/gopath/src/github.com/caddyserver/caddy: no such file or directory
Cloning into '/go/src/github.com/caddyserver/builds'...
fatal: could not read Username for 'https://github.com': No such device or address
stat build.go: no such file or directory
```

I presume something has been changed in mholt/certmagic since 1.0.3 tag release (single or few submodules included by client.go?), but can't figure out where to fix it.