feat: Support non-complicated x86_64 builds.

[patricklodder]

Post-merge feature after #14:

If we can find a way to give the TARGETPLATFORM build argument a default value of linux/amd64, we can make the build process simpler.

From: #14 (comment)

[AbcSxyZ]

The idea is to detect the architecture inside the Dockerfile, right ? Not to enable the user to use --build-arg with some trick ?

Possibly dpkg or uname may do the job. I would love to see output of the following commands:

• dpkg --print-architecture
• dpkg-architecture --query DEB_BUILD_GNU_TYPE
• dpkg-architecture --query DEB_BUILD_MULTIARCH
• dpkg-architecture --query DEB_BUILD_ARCH
• uname -m

For architectures amd64, arm64, armv7 & i386.

Architecture	--print-architecture	DEB_BUILD_GNU_TYPE	DEB_BUILD_MULTIARCH	DEB_BUILD_ARCH	uname -m
amd64	amd64	x86_64-linux-gnu	x86_64-linux-gnu	amd64	x86_64
arm64	arm64	aarch64-linux-gnu	aarch64-linux-gnu	arm64	aarch64
armv7	armhf	arm-linux-gnueabihf	arm-linux-gnueabihf	armhf	armv7l
i386	i386	i686-linux-gnu	i386-linux-gnu	i386	x86_64

Not sure how I can get other value, if anyone has any missing arch and can share some outputs, I will also try to investigate with Docker and Internet if I could find it.

Idk if there are any other commands who can retrieve architecture, if someone has a suggestion.

Edit: Add uname -m as commands to test.

Edit 2: Add all architectures, by printing commands when building images with qemu/buildx. See outputs in GitHub Action logs.

[AbcSxyZ]

With the research result, what comes to my mind is using dpkg --print-architecture and doing something like that:

RUN set -ex && TARGETPLATFORM=$(dpkg --print-architecture) && \
    if [ "${TARGETPLATFORM}" = "amd64" ]; then TARGETPLATFORM=x86_64-linux-gnu; fi \
    && if [ "${TARGETPLATFORM}" = "arm64" ]; then TARGETPLATFORM=aarch64-linux-gnu; fi \
    && if [ "${TARGETPLATFORM}" = "armhf" ]; then TARGETPLATFORM=arm-linux-gnueabihf; fi \
    && if [ "${TARGETPLATFORM}" = "i386" ]; then TARGETPLATFORM=i686-pc-linux-gnu; fi \
    && wget https://github.com/dogecoin/dogecoin/releases/download/v${VERSION}/dogecoin-${VERSION}-${TARGETPLATFORM}.tar.gz

We're having cross architecture build without buildx dependency :)

Edit: I installed dpkg-dev, but maybe it's not even needed, I'm verifying ! [It's working, removing dependency]