Is this a .net issue or a buildx issue ?

Building for AMD/64 works fine :

d:\Dev\dockertesting>docker buildx build --platform linux/amd64 -t ramblinggeekuk/dockertesting --push . [+] Building 211.7s (15/15) FINISHED => [internal] load build definition from Dockerfile 0.2s => => transferring dockerfile: 32B 0.0s => [internal] load .dockerignore 0.3s => => transferring context: 2B 0.0s => [internal] load metadata for mcr.microsoft.com/dotnet/core/aspnet:2.2 1.1s => [internal] load metadata for mcr.microsoft.com/dotnet/core/sdk:2.2 1.2s => [build-env 1/6] FROM mcr.microsoft.com/dotnet/core/sdk:2.2@sha256:b4c25c26dc73f498073fcdb4aefe1677 110.2s => => resolve mcr.microsoft.com/dotnet/core/sdk:2.2@sha256:b4c25c26dc73f498073fcdb4aefe167793eb3a8c79ef 0.0s => => sha256:b4c25c26dc73f498073fcdb4aefe167793eb3a8c79effa76df768006b5c345b8 2.19kB / 2.19kB 0.0s => => sha256:36add2b62b779b538e3062839e1978d78e12e26ec2214940e9043924a29890c0 1.80kB / 1.80kB 0.0s => => sha256:ade3808af4d74b6181b7412b73314b5806fa39d140def18c6ee1cdbcb3ed41b1 300.69MB / 300.69MB 40.3s => => sha256:52c7fe5918815504427b3168845267e876464f8b010ccc09d0f61eb67dd6a17e 4.41kB / 4.41kB 0.0s => => sha256:dbdc36973392a980d56b8fab63383ae44582f6502001d8bbdd543aa3bf1d746e 10.79MB / 10.79MB 9.3s => => sha256:aaef3e0262580b9032fc6741fb099c7313834c7cf332500901e87ceeb38ac153 50.07MB / 50.07MB 58.7s => => sha256:a4d8138d0f6b5a441aaa533faf5fe0c3996a6ca42643c46f4402c7e8bda53742 45.34MB / 45.34MB 53.0s => => sha256:f59d6d019dd5b8398eb8d794e3fafe31f9411cc99a71dabfa587bf732b4a7385 4.34MB / 4.34MB 62.4s => => sha256:f62345fbba0dbbb77ba8aca5b81a4f0d8ec16c5d540def66c0b8e8d6492fa444 13.25MB / 13.25MB 59.3s => => sha256:373065ab5fafec0e8bcfd74485dcd728f40b80800867c553e80c7cd92cd5d504 173.83MB / 173.83MB 79.8s => => unpacking mcr.microsoft.com/dotnet/core/sdk:2.2@sha256:b4c25c26dc73f498073fcdb4aefe167793eb3a8c7 29.0s => [stage-1 1/3] FROM mcr.microsoft.com/dotnet/core/aspnet:2.2@sha256:b18d512d00aff0937699014a9ba44234 54.5s => => resolve mcr.microsoft.com/dotnet/core/aspnet:2.2@sha256:b18d512d00aff0937699014a9ba44234692ce424c 0.0s => => sha256:b18d512d00aff0937699014a9ba44234692ce424c70248bedaa5a60972d77327 2.19kB / 2.19kB 0.0s => => sha256:8bd16a07ec8b72f4131a1747ff479048db65b48f54ae2ced1ffb1b42798c952e 1.16kB / 1.16kB 0.0s => => sha256:318149b63beb70e442e84e530f4472f9354e3906874c35be2ba5045b5f7a8c7a 4.06kB / 4.06kB 0.0s => => sha256:fc7181108d403205fda45b28dbddfa1cf07e772fa41244e44f53a341b8b1893d 22.49MB / 22.49MB 27.7s => => sha256:2c86df27317feb8a2806928aa12f27e6c580894e0cb844cb25aaed1420964e3d 17.69MB / 17.69MB 40.7s => => sha256:66dd687a6ad17486c0e3bc4e3c3690cefb7de9ad55f654e65cf657016ed4194c 2.98MB / 2.98MB 41.5s => => sha256:a7638d93f1fe40e3393bfb685305ce5022179c288f5b2a717978ccae465b4d7a 62.13MB / 62.13MB 48.1s => => unpacking mcr.microsoft.com/dotnet/core/aspnet:2.2@sha256:b18d512d00aff0937699014a9ba44234692ce42 5.2s => [internal] load build context 0.5s => => transferring context: 6.63kB 0.0s => [stage-1 2/3] WORKDIR /app 0.4s => [build-env 2/6] WORKDIR /app 0.2s => [build-env 3/6] COPY *.csproj ./ 0.7s => [build-env 4/6] RUN dotnet restore 10.7s => [build-env 5/6] COPY . ./ 0.4s => [build-env 6/6] RUN dotnet publish -c Release -o out 3.6s => [stage-1 3/3] COPY --from=build-env /app/out . 0.3s => exporting to image 83.5s => => exporting layers 1.0s => => exporting manifest sha256:6ac874b02ae2ce6c86c5d79290a04694778b2f86ff787285650c11dce4b2a37e 0.2s => => exporting config sha256:e9625bb3b3e783bcb6f6b7dd8b3ad4a1f090a1156be3bf237d5d4b7c8f97ebcc 0.2s => => pushing layers 81.3s => => pushing manifest for docker.io/ramblinggeekuk/dockertesting:latest 0.6s

Building for linux/arm/v7 - fails…

‘d:\Dev\dockertesting>docker buildx build --platform linux/arm/v7 -t ramblinggeekuk/dockertesting --push . [+] Building 112.6s (11/14) => [internal] load .dockerignore 0.2s => => transferring context: 2B 0.0s => [internal] load build definition from Dockerfile 0.3s => => transferring dockerfile: 443B 0.0s => [internal] load metadata for mcr.microsoft.com/dotnet/core/aspnet:2.2 1.5s => [internal] load metadata for mcr.microsoft.com/dotnet/core/sdk:2.2 1.6s => [build-env 1/6] FROM mcr.microsoft.com/dotnet/core/sdk:2.2@sha256:b4c25c26dc73f498073fcdb4aefe1677 102.6s => => resolve mcr.microsoft.com/dotnet/core/sdk:2.2@sha256:b4c25c26dc73f498073fcdb4aefe167793eb3a8c79ef 0.0s => => sha256:b4c25c26dc73f498073fcdb4aefe167793eb3a8c79effa76df768006b5c345b8 2.19kB / 2.19kB 0.0s => => sha256:27c2d2f8b92b964c1e3f4de6c8025b1f0362a1f3436118d77b3dbfa921cfd9c9 1.80kB / 1.80kB 0.0s => => sha256:493f51ba80c0d5fd46ea25516eb221089190b416d5a8cc2c898517dea68519a4 4.91kB / 4.91kB 0.0s => => sha256:e06d849c15a63e2cf30d5c5af0d9aa87b2f7c6cbfe0e8c3e351fa4c5d4666d11 300.71MB / 300.71MB 44.8s => => sha256:41835060b113803e2ca628a32805c2e1178fe441b81d3e77427749fec4de06e9 9.49MB / 9.49MB 45.9s => => sha256:da770cd5eae6caeefe9468e318964be31036c06e729c2d983756906ede859b17 46.39MB / 46.39MB 51.2s => => sha256:582caf5d2e7bf5e75a96afc2254a97f6e86ad72c8815429ada61280467cc6d6f 3.92MB / 3.92MB 45.0s => => sha256:dd04b2ffc5474ba8df46350a273baaf841243fda01cfe05d3e5429e4ecc9bb19 144.38MB / 144.38MB 73.9s => => sha256:fa48f739865746afb4020d2d370105be51d23dd6ad6faa8663e1365b607d46c2 13.04MB / 13.04MB 52.3s => => sha256:dcb61f1d45657be196f648f75a07805b856fb8f4aebb61138c03c12e2919ee9e 42.08MB / 42.08MB 57.5s => => unpacking mcr.microsoft.com/dotnet/core/sdk:2.2@sha256:b4c25c26dc73f498073fcdb4aefe167793eb3a8c7 27.0s => [stage-1 1/3] FROM mcr.microsoft.com/dotnet/core/aspnet:2.2@sha256:b18d512d00aff0937699014a9ba44234 18.5s => => resolve mcr.microsoft.com/dotnet/core/aspnet:2.2@sha256:b18d512d00aff0937699014a9ba44234692ce424c 0.0s => => sha256:b18d512d00aff0937699014a9ba44234692ce424c70248bedaa5a60972d77327 2.19kB / 2.19kB 0.0s => => sha256:9ad51bcfeeb6e58218f23fb1f4c5229b39008cc245c9df1fcf8c9330c18a2acb 1.16kB / 1.16kB 0.0s => => sha256:8b7eead4e00d6228dbbf945848d78b43580687575eb8cba1d7a2b11129186f77 4.07kB / 4.07kB 0.0s => => sha256:a51e654c7ec5bf1fd3f38645d4bc8aa40f86ca7803d70031a9828ae65e3b67ae 63.47MB / 63.47MB 8.9s => => sha256:2eead4197fac409644fd8aaf115559d6383b0d56f1ad04d7116aaabbcbea8bed 19.28MB / 19.28MB 10.3s => => sha256:9358a462710e1891aec7076e8674e6f522f08a9f3624dc1f55554c2fc7cb99ea 16.30MB / 16.30MB 12.0s => => sha256:14144450932b5358107e71ebcd25ec878cb799ccc75ec39386e374d0dad903b3 2.88MB / 2.88MB 12.2s => => unpacking mcr.microsoft.com/dotnet/core/aspnet:2.2@sha256:b18d512d00aff0937699014a9ba44234692ce42 4.5s => [internal] load build context 0.2s => => transferring context: 7.04kB 0.0s => [stage-1 2/3] WORKDIR /app 0.2s => [build-env 2/6] WORKDIR /app 0.3s => [build-env 3/6] COPY *.csproj ./ 0.4s => ERROR [build-env 4/6] RUN dotnet restore 7.0s’