Broken image for linux/arm/v7

Hi all,

I discovered a bit of a problem with the linux/arm/v7 version of the Rust image - attempting to build Rust projects with it breaks in various mysterious ways with errors like this:

 > [builder  6/12] RUN cargo install cargo-build-dependencies:                                                                                                                                                                                                                                                                                                                               
#14 0.834     Updating crates.io index                                                                                                                                                                                                                                                                                                                                                       
#14 1.590 warning: spurious network error (2 tries remaining): could not read directory '/usr/local/cargo/registry/index/github.com-1285ae84e5963aae/.git//refs': Value too large for defined data type; class=Os (2)                                                                                                                                                                        
#14 2.087 warning: spurious network error (1 tries remaining): could not read directory '/usr/local/cargo/registry/index/github.com-1285ae84e5963aae/.git//refs': Value too large for defined data type; class=Os (2)                                                                                                                                                                        
#14 2.628 error: failed to fetch `https://github.com/rust-lang/crates.io-index`
#14 2.628 
#14 2.628 Caused by:
#14 2.629   could not read directory '/usr/local/cargo/registry/index/github.com-1285ae84e5963aae/.git//refs': Value too large for defined data type; class=Os (2)
------
failed to solve: rpc error: code = Unknown desc = executor failed running [/bin/sh -c cargo install cargo-build-dependencies]: exit code: 101

I can't say for certain the cause, but I can say that if you install Rust on linux/arm/v7 using the "standard" sh.rustup.rs script, it is also broken, because of this function:

get_bitness() {
    need_cmd head
    # Architecture detection without dependencies beyond coreutils.
    # ELF files start out "\x7fELF", and the following byte is
    #   0x01 for 32-bit and
    #   0x02 for 64-bit.
    # The printf builtin on some shells like dash only supports octal
    # escape sequences, so we use those.
    local _current_exe_head
    _current_exe_head=$(head -c 5 /proc/self/exe )
    if [ "$_current_exe_head" = "$(printf '\177ELF\001')" ]; then
        echo 32
    elif [ "$_current_exe_head" = "$(printf '\177ELF\002')" ]; then
        echo 64
    else
        err "unknown platform bitness"
    fi
}

This fails to set the bitness to 32bit, instead falling through to "unknown platform bitness" (because no /proc/self/exe), resulting in a broken installation of Rust.

If the Docker image build uses that standard rustup script, that's probably why the image is also broken.

Start by inspecting the Docker image build for linux/arm/v7 and how it invokes the standard sh.rustup.rs installer, then reproduce the reported cargo install failure. Done means the linux/arm/v7 image installs Rust correctly and can build Rust projects without the reported architecture or filesystem errors.