Fix rust version in docker build

[JohnnyMorganz]

> [linux/amd64 build 4/4] RUN cargo build --all-features --release:
0.141 error: failed to parse lock file at: /app/Cargo.lock
0.141 
0.141 Caused by:
0.141   lock file version `4` was found, but this version of Cargo does not understand this lock file, perhaps Cargo needs to be updated?
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Dockerfile:6
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   4 |     WORKDIR /app
   5 |     COPY . ./
   6 | >>> RUN cargo build ${CARGO_FLAGS}
   7 |     
   8 |     FROM gcr.io/distroless/cc