Fix docker build warnings when using buildx

The following warnings are fixed in the example flavors + cli
Dockerfile:

```
- StageNameCasing: Stage name 'TOOLKIT' should be lowercase (line 7)
- StageNameCasing: Stage name 'OS' should be lowercase (line 10)
- InvalidDefaultArgInFrom: Default value for ARG
  ${TOOLKIT_REPO}:${VERSION} results in empty or invalid base image name
  (line 7)
- JSONArgsRecommended: JSON arguments recommended for CMD to prevent
  unintended behavior related to OS signals (line 97)
- FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 4)
```

Signed-off-by: Fredrik Lönnegren <fredrik.lonnegren@suse.com>

Dockerfile

@@ -1,7 +1,7 @@
 ARG BASE_OS_IMAGE=registry.opensuse.org/opensuse/tumbleweed
 ARG GO_VERSION=1.22
 
-FROM --platform=$BUILDPLATFORM golang:${GO_VERSION}-alpine as elemental-bin
+FROM --platform=$BUILDPLATFORM golang:${GO_VERSION}-alpine AS elemental-bin
 
 ENV CGO_ENABLED=0
 WORKDIR /src/

examples/blue/Dockerfile

@@ -1,10 +1,10 @@
 # run `make build` to build local/elemental-toolkit image
-ARG TOOLKIT_REPO
-ARG VERSION
-FROM ${TOOLKIT_REPO}:${VERSION} as TOOLKIT
+ARG TOOLKIT_REPO=local/elemental-toolkit
+ARG VERSION=latest
+FROM ${TOOLKIT_REPO}:${VERSION} AS toolkit
 
 # OS base image of our choice
-FROM fedora:40 as OS
+FROM fedora:40 AS os
 ARG REPO
 ARG VERSION
 ENV VERSION=${VERSION}
@@ -51,7 +51,7 @@ RUN dnf install -y \
 RUN mkdir -p /oem /system
 
 # Just add the elemental cli
-COPY --from=TOOLKIT /usr/bin/elemental /usr/bin/elemental
+COPY --from=toolkit /usr/bin/elemental /usr/bin/elemental
 
 # This is for automatic testing purposes, do not do this in production.
 RUN echo "PermitRootLogin yes" > /etc/ssh/sshd_config.d/rootlogin.conf
@@ -70,4 +70,4 @@ RUN echo IMAGE_REPO=\"${REPO}\"             >> /etc/os-release && \
     echo GRUB_ENTRY_NAME=\"Elemental\"      >> /etc/os-release
 
 # Good for validation after the build
-CMD /bin/bash
+CMD ["/bin/bash"]

examples/green/Dockerfile

@@ -1,13 +1,13 @@
 # run `make build` to build local/elemental-toolkit image
-ARG TOOLKIT_REPO
-ARG VERSION
+ARG TOOLKIT_REPO=local/elemental-toolkit
+ARG VERSION=latest
 ARG OS_IMAGE=registry.opensuse.org/opensuse/tumbleweed
 ARG OS_VERSION=latest
 
-FROM ${TOOLKIT_REPO}:${VERSION} AS TOOLKIT
+FROM ${TOOLKIT_REPO}:${VERSION} AS toolkit
 
 # OS base image of our choice
-FROM ${OS_IMAGE}:${OS_VERSION} AS OS
+FROM ${OS_IMAGE}:${OS_VERSION} AS os
 ARG REPO
 ARG VERSION
 ENV REPO=${REPO}
@@ -68,7 +68,7 @@ RUN ARCH=$(uname -m); \
     zypper clean --all
 
 # Just add the elemental cli
-COPY --from=TOOLKIT /usr/bin/elemental /usr/bin/elemental
+COPY --from=toolkit /usr/bin/elemental /usr/bin/elemental
 
 # Enable essential services
 RUN systemctl enable NetworkManager.service && \
@@ -94,4 +94,4 @@ RUN echo IMAGE_REPO=\"${REPO}\"         >> /etc/os-release && \
     echo GRUB_ENTRY_NAME=\"Elemental\" >> /etc/os-release
 
 # Good for validation after the build
-CMD /bin/bash
+CMD ["/bin/bash"]

examples/orange/Dockerfile

@@ -1,10 +1,10 @@
 # run `make build` to build local/elemental-toolkit image
-ARG TOOLKIT_REPO
-ARG VERSION
-FROM ${TOOLKIT_REPO}:${VERSION} as TOOLKIT
+ARG TOOLKIT_REPO=local/elemental-toolkit
+ARG VERSION=latest
+FROM ${TOOLKIT_REPO}:${VERSION} AS toolkit
 
 # OS base image of our choice
-FROM ubuntu:24.04 as OS
+FROM ubuntu:24.04 AS os
 ARG REPO
 ARG VERSION
 ENV VERSION=${VERSION}
@@ -62,11 +62,8 @@ RUN curl -L https://mirrors.edge.kernel.org/pub/linux/utils/kbd/kbd-${KBD}.tar.x
 # Symlink grub2-editenv
 RUN ln -sf /usr/bin/grub-editenv /usr/bin/grub2-editenv
 
-# Create non FHS paths
-RUN mkdir -p /oem /system
-
 # Just add the elemental cli
-COPY --from=TOOLKIT /usr/bin/elemental /usr/bin/elemental
+COPY --from=toolkit /usr/bin/elemental /usr/bin/elemental
 
 # Enable essential services
 RUN systemctl enable systemd-networkd.service
@@ -99,4 +96,4 @@ RUN mkdir -p /usr/lib/elemental/bootloader && \
     cp /usr/lib/shim/mmx64.efi /usr/lib/elemental/bootloader/mmx64.efi
 
 # Good for validation after the build
-CMD /bin/bash
+CMD ["/bin/bash"]