modules directory is missing in build/prod in the first grunt prod run

[ptrunk]

Describe the bug
grunt prod does not publish the the modules directory in the first run.

Running "webpack:web" (webpack) task
98% after emitting SizeLimitsPluginWebpack Bundle Analyzer saved report to /tmp/cyberchef/build/prod/BundleAnalyzerReport.html
Hash: 0fd2ac1ed74258588af3
Version: webpack 4.39.3 / grunt-webpack 3.1.3
Time: 29557ms
Built at: 11/09/2019 11:44:48 AM
                                        Asset      Size  Chunks                    Chunk Names
                              assets/main.css   625 KiB       0  [emitted]  [big]  main
                               assets/main.js  3.39 MiB       0  [emitted]  [big]  main
assets/tesseract/lang-data/eng.traineddata.gz  10.4 MiB          [emitted]  [big]  
      assets/tesseract/tesseract-core.wasm.js  3.11 MiB          [emitted]  [big]  
               assets/tesseract/worker.min.js   103 KiB          [emitted]         
                   images/cook_male-32x32.png  1.59 KiB          [emitted]         
                 images/cyberchef-128x128.png  5.83 KiB          [emitted]         
                      images/file-128x128.png  18.9 KiB          [emitted]         
                        images/file-32x32.png   1.9 KiB          [emitted]         
                           images/fork_me.png  5.33 KiB          [emitted]         
                                   index.html  65.4 KiB          [emitted]         

Running "copy:standalone" (copy) task
Copied 1 file

Running grunt prod again creates the directory as expected

Running "webpack:web" (webpack) task
98% after emitting SizeLimitsPluginWebpack Bundle Analyzer saved report to /tmp/cyberchef/build/prod/BundleAnalyzerReport.html
Hash: e3bfa209189b291c3801
Version: webpack 4.39.3 / grunt-webpack 3.1.3
Time: 43477ms
Built at: 11/09/2019 11:45:48 AM
                                        Asset      Size  Chunks                    Chunk Names
               assets/fonts/Roboto72White.fnt  24.9 KiB          [emitted]         
               assets/fonts/Roboto72White.png  51.5 KiB          [emitted]         
          assets/fonts/RobotoBlack72White.fnt    25 KiB          [emitted]         
          assets/fonts/RobotoBlack72White.png    49 KiB          [emitted]         
           assets/fonts/RobotoMono72White.fnt  10.9 KiB          [emitted]         
           assets/fonts/RobotoMono72White.png  51.3 KiB          [emitted]         
           assets/fonts/RobotoSlab72White.fnt  25.2 KiB          [emitted]         
           assets/fonts/RobotoSlab72White.png    53 KiB          [emitted]         
                              assets/main.css   625 KiB       0  [emitted]  [big]  main
                               assets/main.js  3.39 MiB       0  [emitted]  [big]  main
assets/tesseract/lang-data/eng.traineddata.gz  10.4 MiB          [emitted]  [big]  
      assets/tesseract/tesseract-core.wasm.js  3.11 MiB          [emitted]  [big]  
               assets/tesseract/worker.min.js   103 KiB          [emitted]         
                   images/cook_male-32x32.png  1.59 KiB          [emitted]         
                 images/cyberchef-128x128.png  5.83 KiB          [emitted]         
                      images/file-128x128.png  18.9 KiB          [emitted]         
                        images/file-32x32.png   1.9 KiB          [emitted]         
                           images/fork_me.png  5.33 KiB          [emitted]         
                                   index.html  65.4 KiB          [emitted]         
                         modules/Bletchley.js   174 KiB       1  [emitted]         modules/Bletchley
                            modules/Charts.js   419 KiB       2  [emitted]  [big]  modules/Charts
                           modules/Ciphers.js   508 KiB       3  [emitted]  [big]  modules/Ciphers
                              modules/Code.js  1.54 MiB       4  [emitted]  [big]  modules/Code
                       modules/Compression.js   410 KiB       5  [emitted]  [big]  modules/Compression
                            modules/Crypto.js   694 KiB       6  [emitted]  [big]  modules/Crypto
                              modules/Diff.js   144 KiB       7  [emitted]         modules/Diff
                         modules/Encodings.js  2.85 MiB       8  [emitted]  [big]  modules/Encodings
                           modules/Hashing.js   518 KiB       9  [emitted]  [big]  modules/Hashing
                             modules/Image.js  1010 KiB      10  [emitted]  [big]  modules/Image
                               modules/OCR.js   200 KiB      11  [emitted]         modules/OCR
                               modules/PGP.js  1.02 MiB      12  [emitted]  [big]  modules/PGP
                         modules/PublicKey.js   358 KiB      13  [emitted]  [big]  modules/PublicKey
                             modules/Regex.js   254 KiB      14  [emitted]  [big]  modules/Regex
                         modules/Serialise.js   420 KiB      15  [emitted]  [big]  modules/Serialise
                         modules/Shellcode.js   246 KiB      16  [emitted]  [big]  modules/Shellcode
                               modules/URL.js   141 KiB      17  [emitted]         modules/URL
                         modules/UserAgent.js   142 KiB      18  [emitted]         modules/UserAgent
                              modules/Yara.js  1.41 MiB      19  [emitted]  [big]  modules/Yara

Running "copy:standalone" (copy) task
Copied 1 file

To Reproduce
Follow the compile instructions in the wiki. I use the following Dockerfile to build a container image which can reproduce the problem.

FROM docker.io/node:10.17.0-buster AS builder

ENV CYBERCHEF_VERSION="v9.11.2"
ENV NODE_OPTIONS="--max_old_space_size=2048"

RUN npm install -g grunt-cli

RUN git clone -b ${CYBERCHEF_VERSION} https://github.com/gchq/CyberChef.git /tmp/cyberchef; \
    cd /tmp/cyberchef; \
    npm install; \
    grunt prod

FROM nginx:1.17.5

COPY --from=builder /tmp/cyberchef/build/prod /usr/share/nginx/html

Expected behaviour
build/prod/modules is present after the first grunt prod run.

Environment:

• OS: Linux x86_64
• CyberChef version: v9.11.2

Additional context
Could be related to #871