docker-worker-generator build fails with: gpg: cannot open '/dev/tty'

When building docker-worker-generator, a crash occurs when running the script tasks/70_doc. This seems to have something to do with the gpg command. Here is a stack trace:

...
[00:12:29| 0s] aspell-autobuildhash: processing: en [en_US-w_accents-only].
[00:12:29| 0s] aspell-autobuildhash: processing: en [en_US-wo_accents-only].
[00:12:29| 0s] Processing triggers for libgdk-pixbuf2.0-0:amd64 (2.36.5-2+deb9u2) ...
[00:12:29| 0s] ++ cat
[00:12:29| 0s] + echo '--------------------- end of script /root/INSTALL/tasks.d/60_x11_tools ---------------------'
[00:12:29| 0s] + for script in $(enumerate_tasks | sort -k1 -t':' | cut -f2 -d':')
[00:12:29| 0s] + echo '--------------------- start script /root/INSTALL/tasks.d/70_doc ---------------------'
[00:12:29| 0s] + . /root/INSTALL/tasks.d/70_doc
[00:12:29| 0s] --------------------- end of script /root/INSTALL/tasks.d/60_x11_tools ---------------------
[00:12:29| 0s] --------------------- start script /root/INSTALL/tasks.d/70_doc ---------------------
[00:12:29| 0s] +++ which node
[00:12:29| 0s] ++ [[ -z /usr/bin/node ]]
[00:12:29| 0s] ++ curl -v --connect-timeout 60 --retry 3 -sSL https://rvm.io/mpapis.asc
[00:12:29| 0s] ++ gpg --import -
[00:12:29| 0s] gpg: directory '/root/.gnupg' created
[00:12:29| 0s] gpg: keybox '/root/.gnupg/pubring.kbx' created
[00:12:29| 0s] * Trying 151.101.66.49...
[00:12:29| 0s] * TCP_NODELAY set
[00:12:29| 0s] * Connected to rvm.io (151.101.66.49) port 443 (#0)
[00:12:29| 0s] * ALPN, offering h2
[00:12:29| 0s] * ALPN, offering http/1.1
[00:12:29| 0s] * Cipher selection: ALL:!EXPORT:!EXPORT40:!EXPORT56:!aNULL:!LOW:!RC4:@STRENGTH
[00:12:29| 0s] * successfully set certificate verify locations:
[00:12:29| 0s] * CAfile: /etc/ssl/certs/ca-certificates.crt
[00:12:29| 0s] CApath: /etc/ssl/certs
[00:12:29| 0s] * TLSv1.2 (OUT), TLS header, Certificate Status (22):
[00:12:29| 0s] } [5 bytes data]
[00:12:29| 0s] * TLSv1.2 (OUT), TLS handshake, Client hello (1):
[00:12:29| 0s] } [512 bytes data]
[00:12:29| 0s] * TLSv1.2 (IN), TLS handshake, Server hello (2):
[00:12:29| 0s] { [102 bytes data]
[00:12:29| 0s] * TLSv1.2 (IN), TLS handshake, Certificate (11):
[00:12:29| 0s] { [5210 bytes data]
[00:12:29| 0s] * TLSv1.2 (IN), TLS handshake, Server key exchange (12):
[00:12:29| 0s] { [333 bytes data]
[00:12:29| 0s] * TLSv1.2 (IN), TLS handshake, Server finished (14):
[00:12:29| 0s] { [4 bytes data]
[00:12:29| 0s] * TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
[00:12:29| 0s] } [70 bytes data]
[00:12:29| 0s] * TLSv1.2 (OUT), TLS change cipher, Client hello (1):
[00:12:29| 0s] } [1 bytes data]
[00:12:29| 0s] * TLSv1.2 (OUT), TLS handshake, Finished (20):
[00:12:29| 0s] } [16 bytes data]
[00:12:29| 0s] * TLSv1.2 (IN), TLS change cipher, Client hello (1):
[00:12:29| 0s] { [1 bytes data]
[00:12:29| 0s] * TLSv1.2 (IN), TLS handshake, Finished (20):
[00:12:29| 0s] { [16 bytes data]
[00:12:29| 0s] * SSL connection using TLSv1.2 / ECDHE-RSA-AES128-GCM-SHA256
[00:12:29| 0s] * ALPN, server accepted to use h2
[00:12:29| 0s] * Server certificate:
[00:12:29| 0s] * subject: C=US; ST=California; L=San Francisco; O=Fastly, Inc.; CN=q2.shared.global.fastly.net
[00:12:29| 0s] * start date: Nov 20 11:21:13 2018 GMT
[00:12:29| 0s] * expire date: Oct 11 20:12:15 2019 GMT
[00:12:29| 0s] * subjectAltName: host "rvm.io" matched cert's "rvm.io"
[00:12:29| 0s] * issuer: C=BE; O=GlobalSign nv-sa; CN=GlobalSign CloudSSL CA - SHA256 - G3
[00:12:29| 0s] * SSL certificate verify ok.
[00:12:29| 0s] * Using HTTP2, server supports multi-use
[00:12:29| 0s] * Connection state changed (HTTP/2 confirmed)
[00:12:29| 0s] * Copying HTTP/2 data in stream buffer to connection buffer after upgrade: len=0
[00:12:29| 0s] } [5 bytes data]
[00:12:29| 0s] * Using Stream ID: 1 (easy handle 0x558df41badc0)
[00:12:29| 0s] } [5 bytes data]
[00:12:29| 0s] > GET /mpapis.asc HTTP/1.1
[00:12:29| 0s] > Host: rvm.io
[00:12:29| 0s] > User-Agent: curl/7.52.1
[00:12:29| 0s] > Accept: /
[00:12:29| 0s] >
[00:12:29| 0s] { [5 bytes data]
[00:12:29| 0s] * Connection state changed (MAX_CONCURRENT_STREAMS updated)!
[00:12:29| 0s] } [5 bytes data]
[00:12:29| 0s] < HTTP/2 200
[00:12:29| 0s] < server: nginx/1.10.3 (Ubuntu)
[00:12:29| 0s] < content-type: application/octet-stream
[00:12:29| 0s] < last-modified: Thu, 09 Nov 2017 13:43:02 GMT
[00:12:29| 0s] < etag: "5a045b66-9234"
[00:12:29| 0s] < strict-transport-security: max-age=63072000; includeSubDomains
[00:12:29| 0s] < x-frame-options: DENY
[00:12:29| 0s] < x-content-type-options: nosniff
[00:12:29| 0s] < accept-ranges: bytes
[00:12:29| 0s] < date: Thu, 22 Nov 2018 23:06:32 GMT
[00:12:29| 0s] < via: 1.1 varnish
[00:12:29| 0s] < age: 3085
[00:12:29| 0s] < x-served-by: cache-syd18933-SYD
[00:12:29| 0s] < x-cache: HIT
[00:12:29| 0s] < x-cache-hits: 1
[00:12:29| 0s] < x-timer: S1542927993.916234,VS0,VE5
[00:12:29| 0s] < content-length: 37428
[00:12:29| 0s] <
[00:12:29| 0s] { [2465 bytes data]
[00:12:29| 0s] * Curl_http_done: called premature == 0
[00:12:29| 0s] * Connection #0 to host rvm.io left intact
[00:12:29| 0s] gpg: cannot open '/dev/tty': No such device or address
[00:12:29| 0s] ++ crash
[00:12:29| 0s] ++ set +x
[00:12:29| 0s] -------------------------
[00:12:29| 0s] | CRASH HANDLER TRIGGERED |
[00:12:29| 0s] -------------------------
The command '/bin/sh -c /root/INSTALL/setup_image.sh' returned a non-zero code: 1