docker build e2e error: There is indeed no arbitrum branch in light-client

[XuHugo]

Thanks for filing a bug report :-)

I build docker and can't find the arbitrum branch; There is indeed no arbitrum branch in light-client

 => CANCELED [contract-builder 5/5] RUN /opt/contracts/venv/bin/pip install git+https://github.com/raiden-network/raiden-contracts@v1.0.0rc4    1.1s
 => ERROR [raiden-builder 4/8] RUN git checkout arbitrum                                                                                        1.1s
 => CANCELED [stage-3  2/27] RUN apt-get update   && apt-get install -y --no-install-recommends curl supervisor python3-virtualenv libgtk2.0-0  1.1s
 => CANCELED [synapse-builder 4/6] RUN /opt/synapse/venv/bin/pip install   "matrix-synapse[postgres,redis]==v1.35.1"   psycopg2   coincurve     1.1s
------
 > [raiden-builder 4/8] RUN git checkout arbitrum:
1.038 error: pathspec 'arbitrum' did not match any file(s) known to git
------

 Dockerfile:24
--------------------
5 warnings found (use docker --debug to expand):
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 42)
 - SecretsUsedInArgOrEnv: Do not use ARG or ENV instructions for sensitive data (ENV "PASSWORD_FILE") (line 78)
 - SecretsUsedInArgOrEnv: Do not use ARG or ENV instructions for sensitive data (ENV "PASSWORD") (line 79)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 17)
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 30)
  22 |
  23 |     RUN git clone --recurse-submodules https://github.com/raiden-network/light-client.git .
  24 | >>> RUN git checkout ${RAIDEN_VERSION}
  25 |     RUN git submodule update
  26 |     RUN yarn install --frozen-lockfile
--------------------
ERROR: failed to solve: process "/bin/sh -c git checkout ${RAIDEN_VERSION}" did not complete successfully: exit code: 1

Steps to Reproduce

1. docker build -t raiden-priv-eth:1 .

Expected Result

Actual Result

Additional Infos (Optional)

- Browser & version
- Browser console errors
- Device / OS

[XuHugo]

@ulope @netcriptus @palango @karlb ,Please have a look