azure: docker: fix ARM builds by replacing gosu(1)

Our nightly builds are currently failing due to our ARM-based jobs.
These jobs crash immediately when entering the Docker container with a
exception thrown by Go's language runtime. As we're able to successfully
builds the Docker images in previous steps, it's unlikely to be a bug in
Docker itself. Instead, this exception is thrown by gosu(1), which is a
Go-based utility to drop privileges and run by our entrypoint.

Fix the issue by dropping gosu(1) in favor of sudo(1).

azure-pipelines/docker/bionic

@@ -7,7 +7,6 @@ RUN apt-get update && \
         curl \
         gcc \
         git \
-        gosu \
         libcurl4-openssl-dev \
         libpcre3-dev \
         libssh2-1-dev \
@@ -19,6 +18,7 @@ RUN apt-get update && \
         openssl \
         pkgconf \
         python \
+        sudo \
         valgrind \
         && \
     rm -rf /var/lib/apt/lists/*

azure-pipelines/docker/entrypoint.sh

 #!/bin/bash -e
 useradd --shell /bin/bash libgit2
-chown -R $(id -u libgit2) /home/libgit2
-exec gosu libgit2 "$@"
+chown --recursive libgit2:libgit2 /home/libgit2
+exec sudo --preserve-env --user=libgit2 "$@"

azure-pipelines/docker/xenial

@@ -8,7 +8,6 @@ RUN apt-get update && \
         curl \
         gcc \
         git \
-        gosu \
         krb5-user \
         libcurl4-gnutls-dev \
         libgcrypt20-dev \
@@ -23,6 +22,7 @@ RUN apt-get update && \
         openssl \
         pkgconf \
         python \
+        sudo \
         valgrind \
         && \
     rm -rf /var/lib/apt/lists/*